python_doc_md/Beginner/whatsnew/3.4.md

# What\'s New In Python 3.4

Author
:   R. David Murray \<rdmurray@bitdance.com\> (Editor)

> \* Anyone can add text to this document, but the maintainer reserves the right to rewrite any additions. In particular, for obscure or esoteric features, the maintainer may reduce any addition to a simple reference to the new documentation rather than explaining the feature inline.
>
> \* While the maintainer will periodically go through Misc/NEWS and add changes, it\'s best not to rely on this. We know from experience that any changes that aren\'t in the What\'s New documentation around the time of the original release will remain largely unknown to the community for years, even if they\'re added later. We also know from experience that other priorities can arise, and the maintainer will run out of time to do updates \-- in such cases, end users will be much better served by partial notifications that at least give a hint about new features to investigate.
>
> \* This is not a complete list of every single change; completeness is the purpose of Misc/NEWS. The What\'s New should focus on changes that are visible to Python *users* and that *require* a feature release (i.e. most bug fixes should only be recorded in Misc/NEWS)
>
> \* PEPs should not be marked Final until they have an entry in What\'s New. A placeholder entry that is just a section header and a link to the PEP (e.g \"`397`{.interpreted-text role="pep"} has been implemented\") is acceptable. If a PEP has been implemented and noted in What\'s New, don\'t forget to mark it as Final!
>
> \* If you want to draw your new text to the attention of the maintainer, add \'XXX\' to the beginning of the paragraph or section.
>
> \* It\'s OK to add just a very brief note about a change. For example: \"The `~socket.transmogrify()`{.interpreted-text role="ref"} function was added to the `socket`{.interpreted-text role="mod"} module.\" The maintainer will research the change and write the necessary text (if appropriate). The advantage of doing this is that even if no more descriptive text is ever added, readers will at least have a notification that the new feature exists and a link to the relevant documentation.
>
> \* You can comment out your additions if you like, but it\'s not necessary (especially when a final release is some months away).
>
> \* Credit the author of a patch or bugfix. Just the name is sufficient; the e-mail address isn\'t necessary.
>
> - It\'s helpful to add the bug/patch number as a comment:
>
> The `~socket.transmogrify()`{.interpreted-text role="ref"} function was added to the `socket`{.interpreted-text role="mod"} module. (Contributed by P.Y. Developer in `12345`{.interpreted-text role="issue"}.)
>
> This saves the maintainer the effort of going through the Mercurial log when researching a change.
>
> \* Cross referencing tip: `mod.attr`{.interpreted-text role="ref"} will display as `mod.attr`, while `~mod.attr`{.interpreted-text role="ref"} will display as `attr`.

This article explains the new features in Python 3.4, compared to 3.3. Python 3.4 was released on March 16, 2014. For full details, see the [changelog](https://docs.python.org/3.4/whatsnew/changelog.html).

::: seealso
`429`{.interpreted-text role="pep"} \-- Python 3.4 Release Schedule
:::

## Summary \-- Release Highlights

New syntax features:

- No new syntax features were added in Python 3.4.

Other new features:

- `pip should always be available <whatsnew-pep-453>`{.interpreted-text role="ref"} (`453`{.interpreted-text role="pep"}).
- `Newly created file descriptors are non-inheritable <whatsnew-pep-446>`{.interpreted-text role="ref"} (`446`{.interpreted-text role="pep"}).
- command line option for `isolated mode <whatsnew-isolated-mode>`{.interpreted-text role="ref"} (`16499`{.interpreted-text role="issue"}).
- `improvements in the handling of codecs <codec-handling-improvements>`{.interpreted-text role="ref"} that are not text encodings (multiple issues).
- `A ModuleSpec Type <whatsnew-pep-451>`{.interpreted-text role="ref"} for the Import System (`451`{.interpreted-text role="pep"}). (Affects importer authors.)
- The `marshal`{.interpreted-text role="mod"} format has been made `more compact and efficient
  <whatsnew-marshal-3>`{.interpreted-text role="ref"} (`16475`{.interpreted-text role="issue"}).

New library modules:

- `asyncio`{.interpreted-text role="mod"}: `New provisional API for asynchronous IO
  <whatsnew-asyncio>`{.interpreted-text role="ref"} (`3156`{.interpreted-text role="pep"}).
- `ensurepip`{.interpreted-text role="mod"}: `Bootstrapping the pip installer <whatsnew-ensurepip>`{.interpreted-text role="ref"} (`453`{.interpreted-text role="pep"}).
- `enum`{.interpreted-text role="mod"}: `Support for enumeration types <whatsnew-enum>`{.interpreted-text role="ref"} (`435`{.interpreted-text role="pep"}).
- `pathlib`{.interpreted-text role="mod"}: `Object-oriented filesystem paths <whatsnew-pathlib>`{.interpreted-text role="ref"} (`428`{.interpreted-text role="pep"}).
- `selectors`{.interpreted-text role="mod"}: `High-level and efficient I/O multiplexing
  <whatsnew-selectors>`{.interpreted-text role="ref"}, built upon the `select`{.interpreted-text role="mod"} module primitives (part of `3156`{.interpreted-text role="pep"}).
- `statistics`{.interpreted-text role="mod"}: A basic `numerically stable statistics library
  <whatsnew-statistics>`{.interpreted-text role="ref"} (`450`{.interpreted-text role="pep"}).
- `tracemalloc`{.interpreted-text role="mod"}: `Trace Python memory allocations
  <whatsnew-tracemalloc>`{.interpreted-text role="ref"} (`454`{.interpreted-text role="pep"}).

Significantly improved library modules:

- `Single-dispatch generic functions <whatsnew-singledispatch>`{.interpreted-text role="ref"} in `functools`{.interpreted-text role="mod"} (`443`{.interpreted-text role="pep"}).
- New `pickle`{.interpreted-text role="mod"} `protocol 4 <whatsnew-protocol-4>`{.interpreted-text role="ref"} (`3154`{.interpreted-text role="pep"}).
- `multiprocessing`{.interpreted-text role="mod"} now has `an option to avoid using os.fork
  on Unix <whatsnew-multiprocessing-no-fork>`{.interpreted-text role="ref"} (`8713`{.interpreted-text role="issue"}).
- `email`{.interpreted-text role="mod"} has a new submodule, `~email.contentmanager`{.interpreted-text role="mod"}, and a new `~email.message.Message`{.interpreted-text role="mod"} subclass (`~email.message.EmailMessage`{.interpreted-text role="class"}) that `simplify MIME
  handling <whatsnew_email_contentmanager>`{.interpreted-text role="ref"} (`18891`{.interpreted-text role="issue"}).
- The `inspect`{.interpreted-text role="mod"} and `pydoc`{.interpreted-text role="mod"} modules are now capable of correct introspection of a much wider variety of callable objects, which improves the output of the Python `help`{.interpreted-text role="func"} system.
- The `ipaddress`{.interpreted-text role="mod"} module API has been declared stable

Security improvements:

- `Secure and interchangeable hash algorithm <whatsnew-pep-456>`{.interpreted-text role="ref"} (`456`{.interpreted-text role="pep"}).
- `Make newly created file descriptors non-inheritable <whatsnew-pep-446>`{.interpreted-text role="ref"} (`446`{.interpreted-text role="pep"}) to avoid leaking file descriptors to child processes.
- New command line option for `isolated mode <whatsnew-isolated-mode>`{.interpreted-text role="ref"}, (`16499`{.interpreted-text role="issue"}).
- `multiprocessing`{.interpreted-text role="mod"} now has `an option to avoid using os.fork
  on Unix <whatsnew-multiprocessing-no-fork>`{.interpreted-text role="ref"}. *spawn* and *forkserver* are more secure because they avoid sharing data with child processes.
- `multiprocessing`{.interpreted-text role="mod"} child processes on Windows no longer inherit all of the parent\'s inheritable handles, only the necessary ones.
- A new `hashlib.pbkdf2_hmac`{.interpreted-text role="func"} function provides the [PKCS#5 password-based key derivation function 2](https://en.wikipedia.org/wiki/PBKDF2).
- `TLSv1.1 and TLSv1.2 support <whatsnew-tls-11-12>`{.interpreted-text role="ref"} for `ssl`{.interpreted-text role="mod"}.
- `Retrieving certificates from the Windows system cert store support
  <whatsnew34-win-cert-store>`{.interpreted-text role="ref"} for `ssl`{.interpreted-text role="mod"}.
- `Server-side SNI (Server Name Indication) support
  <whatsnew34-sni>`{.interpreted-text role="ref"} for `ssl`{.interpreted-text role="mod"}.
- The `ssl.SSLContext`{.interpreted-text role="class"} class has a `lot of improvements
  <whatsnew34-sslcontext>`{.interpreted-text role="ref"}.
- All modules in the standard library that support SSL now support server certificate verification, including hostname matching (`!ssl.match_hostname`{.interpreted-text role="func"}) and CRLs (Certificate Revocation lists, see `ssl.SSLContext.load_verify_locations`{.interpreted-text role="func"}).

CPython implementation improvements:

- `Safe object finalization <whatsnew-pep-442>`{.interpreted-text role="ref"} (`442`{.interpreted-text role="pep"}).
- Leveraging `442`{.interpreted-text role="pep"}, in most cases `module globals are no longer set
  to None during finalization <whatsnew-pep-442>`{.interpreted-text role="ref"} (`18214`{.interpreted-text role="issue"}).
- `Configurable memory allocators <whatsnew-pep-445>`{.interpreted-text role="ref"} (`445`{.interpreted-text role="pep"}).
- `Argument Clinic <whatsnew-pep-436>`{.interpreted-text role="ref"} (`436`{.interpreted-text role="pep"}).

Please read on for a comprehensive list of user-facing changes, including many other smaller improvements, CPython optimizations, deprecations, and potential porting issues.

## New Features

### PEP 453: Explicit Bootstrapping of PIP in Python Installations {#whatsnew-pep-453}

#### Bootstrapping pip By Default

The new `ensurepip`{.interpreted-text role="mod"} module (defined in `453`{.interpreted-text role="pep"}) provides a standard cross-platform mechanism to bootstrap the pip installer into Python installations and virtual environments. The version of `pip` included with Python 3.4.0 is `pip` 1.5.4, and future 3.4.x maintenance releases will update the bundled version to the latest version of `pip` that is available at the time of creating the release candidate.

By default, the commands `pipX` and `pipX.Y` will be installed on all platforms (where X.Y stands for the version of the Python installation), along with the `pip` Python package and its dependencies. On Windows and in virtual environments on all platforms, the unversioned `pip` command will also be installed. On other platforms, the system wide unversioned `pip` command typically refers to the separately installed Python 2 version.

The `pyvenv` command line utility and the `venv`{.interpreted-text role="mod"} module make use of the `ensurepip`{.interpreted-text role="mod"} module to make `pip` readily available in virtual environments. When using the command line utility, `pip` is installed by default, while when using the `venv`{.interpreted-text role="mod"} module `venv-api`{.interpreted-text role="ref"} installation of `pip` must be requested explicitly.

For CPython `source builds on POSIX systems <building-python-on-unix>`{.interpreted-text role="ref"}, the `make install` and `make altinstall` commands bootstrap `pip` by default. This behaviour can be controlled through configure options, and overridden through Makefile options.

On Windows and Mac OS X, the CPython installers now default to installing `pip` along with CPython itself (users may opt out of installing it during the installation process). Window users will need to opt in to the automatic `PATH` modifications to have `pip` available from the command line by default, otherwise it can still be accessed through the Python launcher for Windows as `py -m pip`.

As `discussed in the PEP <0453#recommendations-for-downstream-distributors>`{.interpreted-text role="pep"} platform packagers may choose not to install these commands by default, as long as, when invoked, they provide clear and simple directions on how to install them on that platform (usually using the system package manager).

:::: note
::: title
Note
:::

To avoid conflicts between parallel Python 2 and Python 3 installations, only the versioned `pip3` and `pip3.4` commands are bootstrapped by default when `ensurepip` is invoked directly - the `--default-pip` option is needed to also request the unversioned `pip` command. `pyvenv` and the Windows installer ensure that the unqualified `pip` command is made available in those environments, and `pip` can always be invoked via the `-m` switch rather than directly to avoid ambiguity on systems with multiple Python installations.
::::

#### Documentation Changes

As part of this change, the `installing-index`{.interpreted-text role="ref"} and `distributing-index`{.interpreted-text role="ref"} sections of the documentation have been completely redesigned as short getting started and FAQ documents. Most packaging documentation has now been moved out to the Python Packaging Authority maintained [Python Packaging User Guide](https://packaging.python.org) and the documentation of the individual projects.

However, as this migration is currently still incomplete, the legacy versions of those guides remaining available as `install-index`{.interpreted-text role="ref"} and `setuptools-index`{.interpreted-text role="ref"}.

::: seealso

`453`{.interpreted-text role="pep"} \-- Explicit bootstrapping of pip in Python installations

:   PEP written by Donald Stufft and Nick Coghlan, implemented by Donald Stufft, Nick Coghlan, Martin von Löwis and Ned Deily.
:::

### PEP 446: Newly Created File Descriptors Are Non-Inheritable {#whatsnew-pep-446}

`446`{.interpreted-text role="pep"} makes newly created file descriptors `non-inheritable
<fd_inheritance>`{.interpreted-text role="ref"}. In general, this is the behavior an application will want: when launching a new process, having currently open files also open in the new process can lead to all sorts of hard to find bugs, and potentially to security issues.

However, there are occasions when inheritance is desired. To support these cases, the following new functions and methods are available:

- `os.get_inheritable`{.interpreted-text role="func"}, `os.set_inheritable`{.interpreted-text role="func"}
- `os.get_handle_inheritable`{.interpreted-text role="func"}, `os.set_handle_inheritable`{.interpreted-text role="func"}
- `socket.socket.get_inheritable`{.interpreted-text role="meth"}, `socket.socket.set_inheritable`{.interpreted-text role="meth"}

::: seealso

`446`{.interpreted-text role="pep"} \-- Make newly created file descriptors non-inheritable

:   PEP written and implemented by Victor Stinner.
:::

### Improvements to Codec Handling {#codec-handling-improvements}

Since it was first introduced, the `codecs`{.interpreted-text role="mod"} module has always been intended to operate as a type-neutral dynamic encoding and decoding system. However, its close coupling with the Python text model, especially the type restricted convenience methods on the builtin `str`{.interpreted-text role="class"}, `bytes`{.interpreted-text role="class"} and `bytearray`{.interpreted-text role="class"} types, has historically obscured that fact.

As a key step in clarifying the situation, the `codecs.encode`{.interpreted-text role="meth"} and `codecs.decode`{.interpreted-text role="meth"} convenience functions are now properly documented in Python 2.7, 3.3 and 3.4. These functions have existed in the `codecs`{.interpreted-text role="mod"} module (and have been covered by the regression test suite) since Python 2.4, but were previously only discoverable through runtime introspection.

Unlike the convenience methods on `str`{.interpreted-text role="class"}, `bytes`{.interpreted-text role="class"} and `bytearray`{.interpreted-text role="class"}, the `codecs`{.interpreted-text role="mod"} convenience functions support arbitrary codecs in both Python 2 and Python 3, rather than being limited to Unicode text encodings (in Python 3) or `basestring` \<-\> `basestring` conversions (in Python 2).

In Python 3.4, the interpreter is able to identify the known non-text encodings provided in the standard library and direct users towards these general purpose convenience functions when appropriate:

    >>> b"abcdef".decode("hex")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    LookupError: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs

    >>> "hello".encode("rot13")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    LookupError: 'rot13' is not a text encoding; use codecs.encode() to handle arbitrary codecs

    >>> open("foo.txt", encoding="hex")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    LookupError: 'hex' is not a text encoding; use codecs.open() to handle arbitrary codecs

In a related change, whenever it is feasible without breaking backwards compatibility, exceptions raised during encoding and decoding operations are wrapped in a chained exception of the same type that mentions the name of the codec responsible for producing the error:

    >>> import codecs

    >>> codecs.decode(b"abcdefgh", "hex")
    Traceback (most recent call last):
      File "/usr/lib/python3.4/encodings/hex_codec.py", line 20, in hex_decode
        return (binascii.a2b_hex(input), len(input))
    binascii.Error: Non-hexadecimal digit found

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    binascii.Error: decoding with 'hex' codec failed (Error: Non-hexadecimal digit found)

    >>> codecs.encode("hello", "bz2")
    Traceback (most recent call last):
      File "/usr/lib/python3.4/encodings/bz2_codec.py", line 17, in bz2_encode
        return (bz2.compress(input), len(input))
      File "/usr/lib/python3.4/bz2.py", line 498, in compress
        return comp.compress(data) + comp.flush()
    TypeError: 'str' does not support the buffer interface

    The above exception was the direct cause of the following exception:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: encoding with 'bz2' codec failed (TypeError: 'str' does not support the buffer interface)

Finally, as the examples above show, these improvements have permitted the restoration of the convenience aliases for the non-Unicode codecs that were themselves restored in Python 3.2. This means that encoding binary data to and from its hexadecimal representation (for example) can now be written as:

    >>> from codecs import encode, decode
    >>> encode(b"hello", "hex")
    b'68656c6c6f'
    >>> decode(b"68656c6c6f", "hex")
    b'hello'

The binary and text transforms provided in the standard library are detailed in `binary-transforms`{.interpreted-text role="ref"} and `text-transforms`{.interpreted-text role="ref"}.

(Contributed by Nick Coghlan in `7475`{.interpreted-text role="issue"}, `17827`{.interpreted-text role="issue"}, `17828`{.interpreted-text role="issue"} and `19619`{.interpreted-text role="issue"}.)

### PEP 451: A ModuleSpec Type for the Import System {#whatsnew-pep-451}

`451`{.interpreted-text role="pep"} provides an encapsulation of the information about a module that the import machinery will use to load it (that is, a module specification). This helps simplify both the import implementation and several import-related APIs. The change is also a stepping stone for [several future import-related improvements](https://mail.python.org/pipermail/python-dev/2013-November/130111.html).

The public-facing changes from the PEP are entirely backward-compatible. Furthermore, they should be transparent to everyone but importer authors. Key finder and loader methods have been deprecated, but they will continue working. New importers should use the new methods described in the PEP. Existing importers should be updated to implement the new methods. See the `deprecated-3.4`{.interpreted-text role="ref"} section for a list of methods that should be replaced and their replacements.

### Other Language Changes

Some smaller changes made to the core Python language are:

- Unicode database updated to UCD version 6.3.
- `min`{.interpreted-text role="func"} and `max`{.interpreted-text role="func"} now accept a *default* keyword-only argument that can be used to specify the value they return if the iterable they are evaluating has no elements. (Contributed by Julian Berman in `18111`{.interpreted-text role="issue"}.)
- Module objects are now `weakly referenceable <mod-weakref>`{.interpreted-text role="ref"}.
- Module `__file__` attributes (and related values) should now always contain absolute paths by default, with the sole exception of `__main__.__file__` when a script has been executed directly using a relative path. (Contributed by Brett Cannon in `18416`{.interpreted-text role="issue"}.)
- All the UTF-\* codecs (except UTF-7) now reject surrogates during both encoding and decoding unless the `surrogatepass` error handler is used, with the exception of the UTF-16 decoder (which accepts valid surrogate pairs) and the UTF-16 encoder (which produces them while encoding non-BMP characters). (Contributed by Victor Stinner, Kang-Hao (Kenny) Lu and Serhiy Storchaka in `12892`{.interpreted-text role="issue"}.)
- New German EBCDIC `codec <standard-encodings>`{.interpreted-text role="ref"} `cp273`. (Contributed by Michael Bierenfeld and Andrew Kuchling in `1097797`{.interpreted-text role="issue"}.)
- New Ukrainian `codec <standard-encodings>`{.interpreted-text role="ref"} `cp1125`. (Contributed by Serhiy Storchaka in `19668`{.interpreted-text role="issue"}.)
- `bytes`{.interpreted-text role="class"}.join() and `bytearray`{.interpreted-text role="class"}.join() now accept arbitrary buffer objects as arguments. (Contributed by Antoine Pitrou in `15958`{.interpreted-text role="issue"}.)
- The `int`{.interpreted-text role="class"} constructor now accepts any object that has an `__index__` method for its *base* argument. (Contributed by Mark Dickinson in `16772`{.interpreted-text role="issue"}.)
- Frame objects now have a `~frame.clear`{.interpreted-text role="func"} method that clears all references to local variables from the frame. (Contributed by Antoine Pitrou in `17934`{.interpreted-text role="issue"}.)
- `memoryview`{.interpreted-text role="class"} is now registered as a `Sequence <collections.abc>`{.interpreted-text role="class"}, and supports the `reversed`{.interpreted-text role="func"} builtin. (Contributed by Nick Coghlan and Claudiu Popa in `18690`{.interpreted-text role="issue"} and `19078`{.interpreted-text role="issue"}.)
- Signatures reported by `help`{.interpreted-text role="func"} have been modified and improved in several cases as a result of the introduction of Argument Clinic and other changes to the `inspect`{.interpreted-text role="mod"} and `pydoc`{.interpreted-text role="mod"} modules.
- `~object.__length_hint__`{.interpreted-text role="meth"} is now part of the formal language specification (see `424`{.interpreted-text role="pep"}). (Contributed by Armin Ronacher in `16148`{.interpreted-text role="issue"}.)

## New Modules

### asyncio {#whatsnew-asyncio}

The new `asyncio`{.interpreted-text role="mod"} module (defined in `3156`{.interpreted-text role="pep"}) provides a standard pluggable event loop model for Python, providing solid asynchronous IO support in the standard library, and making it easier for other event loop implementations to interoperate with the standard library and each other.

For Python 3.4, this module is considered a `provisional API`{.interpreted-text role="term"}.

::: seealso

`3156`{.interpreted-text role="pep"} \-- Asynchronous IO Support Rebooted: the \"asyncio\" Module

:   PEP written and implementation led by Guido van Rossum.
:::

### ensurepip {#whatsnew-ensurepip}

The new `ensurepip`{.interpreted-text role="mod"} module is the primary infrastructure for the `453`{.interpreted-text role="pep"} implementation. In the normal course of events end users will not need to interact with this module, but it can be used to manually bootstrap `pip` if the automated bootstrapping into an installation or virtual environment was declined.

`ensurepip`{.interpreted-text role="mod"} includes a bundled copy of `pip`, up-to-date as of the first release candidate of the release of CPython with which it ships (this applies to both maintenance releases and feature releases). `ensurepip` does not access the internet. If the installation has internet access, after `ensurepip` is run the bundled `pip` can be used to upgrade `pip` to a more recent release than the bundled one. (Note that such an upgraded version of `pip` is considered to be a separately installed package and will not be removed if Python is uninstalled.)

The module is named *ensure*pip because if called when `pip` is already installed, it does nothing. It also has an `--upgrade` option that will cause it to install the bundled copy of `pip` if the existing installed version of `pip` is older than the bundled copy.

### enum {#whatsnew-enum}

The new `enum`{.interpreted-text role="mod"} module (defined in `435`{.interpreted-text role="pep"}) provides a standard implementation of enumeration types, allowing other modules (such as `socket`{.interpreted-text role="mod"}) to provide more informative error messages and better debugging support by replacing opaque integer constants with backwards compatible enumeration values.

::: seealso

`435`{.interpreted-text role="pep"} \-- Adding an Enum type to the Python standard library

:   PEP written by Barry Warsaw, Eli Bendersky and Ethan Furman, implemented by Ethan Furman.
:::

### pathlib {#whatsnew-pathlib}

The new `pathlib`{.interpreted-text role="mod"} module offers classes representing filesystem paths with semantics appropriate for different operating systems. Path classes are divided between *pure paths*, which provide purely computational operations without I/O, and *concrete paths*, which inherit from pure paths but also provide I/O operations.

For Python 3.4, this module is considered a `provisional API`{.interpreted-text role="term"}.

::: seealso

`428`{.interpreted-text role="pep"} \-- The pathlib module \-- object-oriented filesystem paths

:   PEP written and implemented by Antoine Pitrou.
:::

### selectors {#whatsnew-selectors}

The new `selectors`{.interpreted-text role="mod"} module (created as part of implementing `3156`{.interpreted-text role="pep"}) allows high-level and efficient I/O multiplexing, built upon the `select`{.interpreted-text role="mod"} module primitives.

### statistics {#whatsnew-statistics}

The new `statistics`{.interpreted-text role="mod"} module (defined in `450`{.interpreted-text role="pep"}) offers some core statistics functionality directly in the standard library. This module supports calculation of the mean, median, mode, variance and standard deviation of a data series.

::: seealso

`450`{.interpreted-text role="pep"} \-- Adding A Statistics Module To The Standard Library

:   PEP written and implemented by Steven D\'Aprano
:::

### tracemalloc {#whatsnew-tracemalloc}

The new `tracemalloc`{.interpreted-text role="mod"} module (defined in `454`{.interpreted-text role="pep"}) is a debug tool to trace memory blocks allocated by Python. It provides the following information:

- Trace where an object was allocated
- Statistics on allocated memory blocks per filename and per line number: total size, number and average size of allocated memory blocks
- Compute the differences between two snapshots to detect memory leaks

::: seealso

`454`{.interpreted-text role="pep"} \-- Add a new tracemalloc module to trace Python memory allocations

:   PEP written and implemented by Victor Stinner
:::

## Improved Modules

### abc

New function `abc.get_cache_token`{.interpreted-text role="func"} can be used to know when to invalidate caches that are affected by changes in the object graph. (Contributed by Łukasz Langa in `16832`{.interpreted-text role="issue"}.)

New class `~abc.ABC`{.interpreted-text role="class"} has `~abc.ABCMeta`{.interpreted-text role="class"} as its meta class. Using `ABC` as a base class has essentially the same effect as specifying `metaclass=abc.ABCMeta`, but is simpler to type and easier to read. (Contributed by Bruno Dupuis in `16049`{.interpreted-text role="issue"}.)

### aifc

The `!getparams`{.interpreted-text role="meth"} method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in `17818`{.interpreted-text role="issue"}.)

`!aifc.open`{.interpreted-text role="func"} now supports the context management protocol: when used in a `with`{.interpreted-text role="keyword"} block, the `!close`{.interpreted-text role="meth"} method of the returned object will be called automatically at the end of the block. (Contributed by Serhiy Storchacha in `16486`{.interpreted-text role="issue"}.)

The `!writeframesraw`{.interpreted-text role="meth"} and `!writeframes`{.interpreted-text role="meth"} methods now accept any `bytes-like object`{.interpreted-text role="term"}. (Contributed by Serhiy Storchaka in `8311`{.interpreted-text role="issue"}.)

### argparse

The `~argparse.FileType`{.interpreted-text role="class"} class now accepts *encoding* and *errors* arguments, which are passed through to `open`{.interpreted-text role="func"}. (Contributed by Lucas Maystre in `11175`{.interpreted-text role="issue"}.)

### audioop

`!audioop`{.interpreted-text role="mod"} now supports 24-bit samples. (Contributed by Serhiy Storchaka in `12866`{.interpreted-text role="issue"}.)

New `!byteswap`{.interpreted-text role="func"} function converts big-endian samples to little-endian and vice versa. (Contributed by Serhiy Storchaka in `19641`{.interpreted-text role="issue"}.)

All `!audioop`{.interpreted-text role="mod"} functions now accept any `bytes-like object`{.interpreted-text role="term"}. Strings are not accepted: they didn\'t work before, now they raise an error right away. (Contributed by Serhiy Storchaka in `16685`{.interpreted-text role="issue"}.)

### base64

The encoding and decoding functions in `base64`{.interpreted-text role="mod"} now accept any `bytes-like object`{.interpreted-text role="term"} in cases where it previously required a `bytes`{.interpreted-text role="class"} or `bytearray`{.interpreted-text role="class"} instance. (Contributed by Nick Coghlan in `17839`{.interpreted-text role="issue"}.)

New functions `~base64.a85encode`{.interpreted-text role="func"}, `~base64.a85decode`{.interpreted-text role="func"}, `~base64.b85encode`{.interpreted-text role="func"}, and `~base64.b85decode`{.interpreted-text role="func"} provide the ability to encode and decode binary data from and to `Ascii85` and the git/mercurial `Base85` formats, respectively. The `a85` functions have options that can be used to make them compatible with the variants of the `Ascii85` encoding, including the Adobe variant. (Contributed by Martin Morrison, the Mercurial project, Serhiy Storchaka, and Antoine Pitrou in `17618`{.interpreted-text role="issue"}.)

### collections

The `.ChainMap.new_child`{.interpreted-text role="meth"} method now accepts an *m* argument specifying the child map to add to the chain. This allows an existing mapping and/or a custom mapping type to be used for the child. (Contributed by Vinay Sajip in `16613`{.interpreted-text role="issue"}.)

### colorsys

The number of digits in the coefficients for the RGB \-\-- YIQ conversions have been expanded so that they match the FCC NTSC versions. The change in results should be less than 1% and may better match results found elsewhere. (Contributed by Brian Landers and Serhiy Storchaka in `14323`{.interpreted-text role="issue"}.)

### contextlib

The new `contextlib.suppress`{.interpreted-text role="class"} context manager helps to clarify the intent of code that deliberately suppresses exceptions from a single statement. (Contributed by Raymond Hettinger in `15806`{.interpreted-text role="issue"} and Zero Piraeus in `19266`{.interpreted-text role="issue"}.)

The new `contextlib.redirect_stdout`{.interpreted-text role="func"} context manager makes it easier for utility scripts to handle inflexible APIs that write their output to `sys.stdout`{.interpreted-text role="data"} and don\'t provide any options to redirect it. Using the context manager, the `sys.stdout`{.interpreted-text role="data"} output can be redirected to any other stream or, in conjunction with `io.StringIO`{.interpreted-text role="class"}, to a string. The latter can be especially useful, for example, to capture output from a function that was written to implement a command line interface. It is recommended only for utility scripts because it affects the global state of `sys.stdout`{.interpreted-text role="data"}. (Contributed by Raymond Hettinger in `15805`{.interpreted-text role="issue"}.)

The `contextlib`{.interpreted-text role="mod"} documentation has also been updated to include a `discussion <single-use-reusable-and-reentrant-cms>`{.interpreted-text role="ref"} of the differences between single use, reusable and reentrant context managers.

### dbm

`dbm.open`{.interpreted-text role="func"} objects now support the context management protocol. When used in a `with`{.interpreted-text role="keyword"} statement, the `close` method of the database object will be called automatically at the end of the block. (Contributed by Claudiu Popa and Nick Coghlan in `19282`{.interpreted-text role="issue"}.)

### dis

Functions `~dis.show_code`{.interpreted-text role="func"}, `~dis.dis`{.interpreted-text role="func"}, `~dis.distb`{.interpreted-text role="func"}, and `~dis.disassemble`{.interpreted-text role="func"} now accept a keyword-only *file* argument that controls where they write their output.

The `dis`{.interpreted-text role="mod"} module is now built around an `~dis.Instruction`{.interpreted-text role="class"} class that provides object oriented access to the details of each individual bytecode operation.

A new method, `~dis.get_instructions`{.interpreted-text role="func"}, provides an iterator that emits the Instruction stream for a given piece of Python code. Thus it is now possible to write a program that inspects and manipulates a bytecode object in ways different from those provided by the `~dis`{.interpreted-text role="mod"} module itself. For example:

    >>> import dis
    >>> for instr in dis.get_instructions(lambda x: x + 1):
    ...     print(instr.opname)
    LOAD_FAST
    LOAD_CONST
    BINARY_ADD
    RETURN_VALUE

The various display tools in the `dis`{.interpreted-text role="mod"} module have been rewritten to use these new components.

In addition, a new application-friendly class `~dis.Bytecode`{.interpreted-text role="class"} provides an object-oriented API for inspecting bytecode in both in human-readable form and for iterating over instructions. The `~dis.Bytecode`{.interpreted-text role="class"} constructor takes the same arguments that `~dis.get_instructions`{.interpreted-text role="func"} does (plus an optional *current_offset*), and the resulting object can be iterated to produce `~dis.Instruction`{.interpreted-text role="class"} objects. But it also has a `~dis.Bytecode.dis`{.interpreted-text role="mod"} method, equivalent to calling `~dis.dis`{.interpreted-text role="mod"} on the constructor argument, but returned as a multi-line string:

    >>> bytecode = dis.Bytecode(lambda x: x + 1, current_offset=3)
    >>> for instr in bytecode:
    ...     print('{} ({})'.format(instr.opname, instr.opcode))
    LOAD_FAST (124)
    LOAD_CONST (100)
    BINARY_ADD (23)
    RETURN_VALUE (83)
    >>> bytecode.dis().splitlines()       # doctest: +NORMALIZE_WHITESPACE
    ['  1           0 LOAD_FAST                0 (x)',
     '      -->     3 LOAD_CONST               1 (1)',
     '              6 BINARY_ADD',
     '              7 RETURN_VALUE']

`~dis.Bytecode`{.interpreted-text role="class"} also has a class method, `~dis.Bytecode.from_traceback`{.interpreted-text role="meth"}, that provides the ability to manipulate a traceback (that is, `print(Bytecode.from_traceback(tb).dis())` is equivalent to `distb(tb)`).

(Contributed by Nick Coghlan, Ryan Kelly and Thomas Kluyver in `11816`{.interpreted-text role="issue"} and Claudiu Popa in `17916`{.interpreted-text role="issue"}.)

New function `~dis.stack_effect`{.interpreted-text role="func"} computes the effect on the Python stack of a given opcode and argument, information that is not otherwise available. (Contributed by Larry Hastings in `19722`{.interpreted-text role="issue"}.)

### doctest

A new `option flag <doctest-options>`{.interpreted-text role="ref"}, `~doctest.FAIL_FAST`{.interpreted-text role="const"}, halts test running as soon as the first failure is detected. (Contributed by R. David Murray and Daniel Urban in `16522`{.interpreted-text role="issue"}.)

The `doctest`{.interpreted-text role="mod"} command line interface now uses `argparse`{.interpreted-text role="mod"}, and has two new options, `-o` and `-f`. `-o` allows `doctest options
<doctest-options>`{.interpreted-text role="ref"} to be specified on the command line, and `-f` is a shorthand for `-o FAIL_FAST` (to parallel the similar option supported by the `unittest`{.interpreted-text role="mod"} CLI). (Contributed by R. David Murray in `11390`{.interpreted-text role="issue"}.)

`doctest`{.interpreted-text role="mod"} will now find doctests in extension module `__doc__` strings. (Contributed by Zachary Ware in `3158`{.interpreted-text role="issue"}.)

### email

`~email.message.Message.as_string`{.interpreted-text role="meth"} now accepts a *policy* argument to override the default policy of the message when generating a string representation of it. This means that `as_string` can now be used in more circumstances, instead of having to create and use a `~email.generator`{.interpreted-text role="mod"} in order to pass formatting parameters to its `flatten` method. (Contributed by R. David Murray in `18600`{.interpreted-text role="issue"}.)

New method `~email.message.Message.as_bytes`{.interpreted-text role="meth"} added to produce a bytes representation of the message in a fashion similar to how `as_string` produces a string representation. It does not accept the *maxheaderlen* argument, but does accept the *unixfrom* and *policy* arguments. The `~email.message.Message`{.interpreted-text role="class"} `~email.message.Message.__bytes__`{.interpreted-text role="meth"} method calls it, meaning that `bytes(mymsg)` will now produce the intuitive result: a bytes object containing the fully formatted message. (Contributed by R. David Murray in `18600`{.interpreted-text role="issue"}.)

The `.Message.set_param`{.interpreted-text role="meth"} message now accepts a *replace* keyword argument. When specified, the associated header will be updated without changing its location in the list of headers. For backward compatibility, the default is `False`. (Contributed by R. David Murray in `18891`{.interpreted-text role="issue"}.)

::: {#whatsnew_email_contentmanager}
A pair of new subclasses of `~email.message.Message`{.interpreted-text role="class"} have been added (`.EmailMessage`{.interpreted-text role="class"} and `.MIMEPart`{.interpreted-text role="class"}), along with a new sub-module, `~email.contentmanager`{.interpreted-text role="mod"} and a new `~email.policy`{.interpreted-text role="mod"} attribute `~email.policy.EmailPolicy.content_manager`{.interpreted-text role="attr"}. All documentation is currently in the new module, which is being added as part of email\'s new `provisional API`{.interpreted-text role="term"}. These classes provide a number of new methods that make extracting content from and inserting content into email messages much easier. For details, see the `~email.contentmanager`{.interpreted-text role="mod"} documentation and the `email-examples`{.interpreted-text role="ref"}. These API additions complete the bulk of the work that was planned as part of the email6 project. The currently provisional API is scheduled to become final in Python 3.5 (possibly with a few minor additions in the area of error handling). (Contributed by R. David Murray in `18891`{.interpreted-text role="issue"}.)
:::

### filecmp

A new `~filecmp.clear_cache`{.interpreted-text role="func"} function provides the ability to clear the `filecmp`{.interpreted-text role="mod"} comparison cache, which uses `os.stat`{.interpreted-text role="func"} information to determine if the file has changed since the last compare. This can be used, for example, if the file might have been changed and re-checked in less time than the resolution of a particular filesystem\'s file modification time field. (Contributed by Mark Levitt in `18149`{.interpreted-text role="issue"}.)

New module attribute `~filecmp.DEFAULT_IGNORES`{.interpreted-text role="const"} provides the list of directories that are used as the default value for the *ignore* parameter of the `~filecmp.dircmp`{.interpreted-text role="func"} function. (Contributed by Eli Bendersky in `15442`{.interpreted-text role="issue"}.)

### functools

The new `~functools.partialmethod`{.interpreted-text role="func"} descriptor brings partial argument application to descriptors, just as `~functools.partial`{.interpreted-text role="func"} provides for normal callables. The new descriptor also makes it easier to get arbitrary callables (including `~functools.partial`{.interpreted-text role="func"} instances) to behave like normal instance methods when included in a class definition. (Contributed by Alon Horev and Nick Coghlan in `4331`{.interpreted-text role="issue"}.)

::: {#whatsnew-singledispatch}
The new `~functools.singledispatch`{.interpreted-text role="func"} decorator brings support for single-dispatch generic functions to the Python standard library. Where object oriented programming focuses on grouping multiple operations on a common set of data into a class, a generic function focuses on grouping multiple implementations of an operation that allows it to work with *different* kinds of data.
:::

::: seealso

`443`{.interpreted-text role="pep"} \-- Single-dispatch generic functions

:   PEP written and implemented by Łukasz Langa.
:::

`~functools.total_ordering`{.interpreted-text role="func"} now supports a return value of `NotImplemented`{.interpreted-text role="data"} from the underlying comparison function. (Contributed by Katie Miller in `10042`{.interpreted-text role="issue"}.)

A pure-python version of the `~functools.partial`{.interpreted-text role="func"} function is now in the stdlib; in CPython it is overridden by the C accelerated version, but it is available for other implementations to use. (Contributed by Brian Thorne in `12428`{.interpreted-text role="issue"}.)

### gc

New function `~gc.get_stats`{.interpreted-text role="func"} returns a list of three per-generation dictionaries containing the collections statistics since interpreter startup. (Contributed by Antoine Pitrou in `16351`{.interpreted-text role="issue"}.)

### glob

A new function `~glob.escape`{.interpreted-text role="func"} provides a way to escape special characters in a filename so that they do not become part of the globbing expansion but are instead matched literally. (Contributed by Serhiy Storchaka in `8402`{.interpreted-text role="issue"}.)

### hashlib

A new `hashlib.pbkdf2_hmac`{.interpreted-text role="func"} function provides the [PKCS#5 password-based key derivation function 2](https://en.wikipedia.org/wiki/PBKDF2). (Contributed by Christian Heimes in `18582`{.interpreted-text role="issue"}.)

The `~hashlib.hash.name`{.interpreted-text role="attr"} attribute of `hashlib`{.interpreted-text role="mod"} hash objects is now a formally supported interface. It has always existed in CPython\'s `hashlib`{.interpreted-text role="mod"} (although it did not return lower case names for all supported hashes), but it was not a public interface and so some other Python implementations have not previously supported it. (Contributed by Jason R. Coombs in `18532`{.interpreted-text role="issue"}.)

### hmac

`hmac`{.interpreted-text role="mod"} now accepts `bytearray` as well as `bytes` for the *key* argument to the `~hmac.new`{.interpreted-text role="func"} function, and the *msg* parameter to both the `~hmac.new`{.interpreted-text role="func"} function and the `~hmac.HMAC.update`{.interpreted-text role="meth"} method now accepts any type supported by the `hashlib`{.interpreted-text role="mod"} module. (Contributed by Jonas Borgström in `18240`{.interpreted-text role="issue"}.)

The *digestmod* argument to the `hmac.new`{.interpreted-text role="func"} function may now be any hash digest name recognized by `hashlib`{.interpreted-text role="mod"}. In addition, the current behavior in which the value of *digestmod* defaults to `MD5` is deprecated: in a future version of Python there will be no default value. (Contributed by Christian Heimes in `17276`{.interpreted-text role="issue"}.)

With the addition of `~hmac.HMAC.block_size`{.interpreted-text role="attr"} and `~hmac.HMAC.name`{.interpreted-text role="attr"} attributes (and the formal documentation of the `~hmac.HMAC.digest_size`{.interpreted-text role="attr"} attribute), the `hmac`{.interpreted-text role="mod"} module now conforms fully to the `247`{.interpreted-text role="pep"} API. (Contributed by Christian Heimes in `18775`{.interpreted-text role="issue"}.)

### html

New function `~html.unescape`{.interpreted-text role="func"} function converts HTML5 character references to the corresponding Unicode characters. (Contributed by Ezio Melotti in `2927`{.interpreted-text role="issue"}.)

`~html.parser.HTMLParser`{.interpreted-text role="class"} accepts a new keyword argument *convert_charrefs* that, when `True`, automatically converts all character references. For backward-compatibility, its value defaults to `False`, but it will change to `True` in a future version of Python, so you are invited to set it explicitly and update your code to use this new feature. (Contributed by Ezio Melotti in `13633`{.interpreted-text role="issue"}.)

The *strict* argument of `~html.parser.HTMLParser`{.interpreted-text role="class"} is now deprecated. (Contributed by Ezio Melotti in `15114`{.interpreted-text role="issue"}.)

### http

`~http.server.BaseHTTPRequestHandler.send_error`{.interpreted-text role="meth"} now accepts an optional additional *explain* parameter which can be used to provide an extended error description, overriding the hardcoded default if there is one. This extended error description will be formatted using the `~http.server.BaseHTTPRequestHandler.error_message_format`{.interpreted-text role="attr"} attribute and sent as the body of the error response. (Contributed by Karl Cow in `12921`{.interpreted-text role="issue"}.)

The `http.server`{.interpreted-text role="mod"} `command line interface <http-server-cli>`{.interpreted-text role="ref"} now has a `-b/--bind` option that causes the server to listen on a specific address. (Contributed by Malte Swart in `17764`{.interpreted-text role="issue"}.)

### idlelib and IDLE

Since idlelib implements the IDLE shell and editor and is not intended for import by other programs, it gets improvements with every release. See `Lib/idlelib/NEWS.txt`{.interpreted-text role="file"} for a cumulative list of changes since 3.3.0, as well as changes made in future 3.4.x releases. This file is also available from the IDLE `Help --> About IDLE`{.interpreted-text role="menuselection"} dialog.

### importlib

The `~importlib.abc.InspectLoader`{.interpreted-text role="class"} ABC defines a new method, `~importlib.abc.InspectLoader.source_to_code`{.interpreted-text role="meth"} that accepts source data and a path and returns a code object. The default implementation is equivalent to `compile(data, path, 'exec', dont_inherit=True)`. (Contributed by Eric Snow and Brett Cannon in `15627`{.interpreted-text role="issue"}.)

`~importlib.abc.InspectLoader`{.interpreted-text role="class"} also now has a default implementation for the `~importlib.abc.InspectLoader.get_code`{.interpreted-text role="meth"} method. However, it will normally be desirable to override the default implementation for performance reasons. (Contributed by Brett Cannon in `18072`{.interpreted-text role="issue"}.)

The `~importlib.reload`{.interpreted-text role="func"} function has been moved from `!imp`{.interpreted-text role="mod"} to `importlib`{.interpreted-text role="mod"} as part of the `!imp`{.interpreted-text role="mod"} module deprecation. (Contributed by Berker Peksag in `18193`{.interpreted-text role="issue"}.)

`importlib.util`{.interpreted-text role="mod"} now has a `~importlib.util.MAGIC_NUMBER`{.interpreted-text role="const"} attribute providing access to the bytecode version number. This replaces the `!get_magic`{.interpreted-text role="func"} function in the deprecated `!imp`{.interpreted-text role="mod"} module. (Contributed by Brett Cannon in `18192`{.interpreted-text role="issue"}.)

New `importlib.util`{.interpreted-text role="mod"} functions `~importlib.util.cache_from_source`{.interpreted-text role="func"} and `~importlib.util.source_from_cache`{.interpreted-text role="func"} replace the same-named functions in the deprecated `!imp`{.interpreted-text role="mod"} module. (Contributed by Brett Cannon in `18194`{.interpreted-text role="issue"}.)

The `importlib`{.interpreted-text role="mod"} bootstrap `.NamespaceLoader`{.interpreted-text role="class"} now conforms to the `.InspectLoader`{.interpreted-text role="class"} ABC, which means that `runpy` and `python -m` can now be used with namespace packages. (Contributed by Brett Cannon in `18058`{.interpreted-text role="issue"}.)

`importlib.util`{.interpreted-text role="mod"} has a new function `~importlib.util.decode_source`{.interpreted-text role="func"} that decodes source from bytes using universal newline processing. This is useful for implementing `.InspectLoader.get_source`{.interpreted-text role="meth"} methods.

`importlib.machinery.ExtensionFileLoader`{.interpreted-text role="class"} now has a `~importlib.machinery.ExtensionFileLoader.get_filename`{.interpreted-text role="meth"} method. This was inadvertently omitted in the original implementation. (Contributed by Eric Snow in `19152`{.interpreted-text role="issue"}.)

### inspect

The `inspect`{.interpreted-text role="mod"} module now offers a basic `command line interface
<inspect-module-cli>`{.interpreted-text role="ref"} to quickly display source code and other information for modules, classes and functions. (Contributed by Claudiu Popa and Nick Coghlan in `18626`{.interpreted-text role="issue"}.)

`~inspect.unwrap`{.interpreted-text role="func"} makes it easy to unravel wrapper function chains created by `functools.wraps`{.interpreted-text role="func"} (and any other API that sets the `__wrapped__` attribute on a wrapper function). (Contributed by Daniel Urban, Aaron Iles and Nick Coghlan in `13266`{.interpreted-text role="issue"}.)

As part of the implementation of the new `enum`{.interpreted-text role="mod"} module, the `inspect`{.interpreted-text role="mod"} module now has substantially better support for custom `__dir__` methods and dynamic class attributes provided through metaclasses. (Contributed by Ethan Furman in `18929`{.interpreted-text role="issue"} and `19030`{.interpreted-text role="issue"}.)

`~inspect.getfullargspec`{.interpreted-text role="func"} and `!getargspec`{.interpreted-text role="func"} now use the `~inspect.signature`{.interpreted-text role="func"} API. This allows them to support a much broader range of callables, including those with `__signature__` attributes, those with metadata provided by argument clinic, `functools.partial`{.interpreted-text role="func"} objects and more. Note that, unlike `~inspect.signature`{.interpreted-text role="func"}, these functions still ignore `__wrapped__` attributes, and report the already bound first argument for bound methods, so it is still necessary to update your code to use `~inspect.signature`{.interpreted-text role="func"} directly if those features are desired. (Contributed by Yury Selivanov in `17481`{.interpreted-text role="issue"}.)

`~inspect.signature`{.interpreted-text role="func"} now supports duck types of CPython functions, which adds support for functions compiled with Cython. (Contributed by Stefan Behnel and Yury Selivanov in `17159`{.interpreted-text role="issue"}.)

### ipaddress

`ipaddress`{.interpreted-text role="mod"} was added to the standard library in Python 3.3 as a `provisional API`{.interpreted-text role="term"}. With the release of Python 3.4, this qualification has been removed: `ipaddress`{.interpreted-text role="mod"} is now considered a stable API, covered by the normal standard library requirements to maintain backwards compatibility.

A new `~ipaddress.IPv4Address.is_global`{.interpreted-text role="attr"} property is `True` if an address is globally routeable. (Contributed by Peter Moody in `17400`{.interpreted-text role="issue"}.)

### logging

The `~logging.handlers.TimedRotatingFileHandler`{.interpreted-text role="class"} has a new *atTime* parameter that can be used to specify the time of day when rollover should happen. (Contributed by Ronald Oussoren in `9556`{.interpreted-text role="issue"}.)

`~logging.handlers.SocketHandler`{.interpreted-text role="class"} and `~logging.handlers.DatagramHandler`{.interpreted-text role="class"} now support Unix domain sockets (by setting *port* to `None`). (Contributed by Vinay Sajip in commit ce46195b56a9.)

`~logging.config.fileConfig`{.interpreted-text role="func"} now accepts a `configparser.RawConfigParser`{.interpreted-text role="class"} subclass instance for the *fname* parameter. This facilitates using a configuration file when logging configuration is just a part of the overall application configuration, or where the application modifies the configuration before passing it to `~logging.config.fileConfig`{.interpreted-text role="func"}. (Contributed by Vinay Sajip in `16110`{.interpreted-text role="issue"}.)

Logging configuration data received from a socket via the `logging.config.listen`{.interpreted-text role="func"} function can now be validated before being processed by supplying a verification function as the argument to the new *verify* keyword argument. (Contributed by Vinay Sajip in `15452`{.interpreted-text role="issue"}.)

### marshal {#whatsnew-marshal-3}

The default `marshal`{.interpreted-text role="mod"} version has been bumped to 3. The code implementing the new version restores the Python2 behavior of recording only one copy of interned strings and preserving the interning on deserialization, and extends this \"one copy\" ability to any object type (including handling recursive references). This reduces both the size of `.pyc` files and the amount of memory a module occupies in memory when it is loaded from a `.pyc` (or `.pyo`) file. (Contributed by Kristján Valur Jónsson in `16475`{.interpreted-text role="issue"}, with additional speedups by Antoine Pitrou in `19219`{.interpreted-text role="issue"}.)

### mmap

mmap objects are now `weakly referenceable <mod-weakref>`{.interpreted-text role="ref"}. (Contributed by Valerie Lambert in `4885`{.interpreted-text role="issue"}.)

### multiprocessing

::: {#whatsnew-multiprocessing-no-fork}
On Unix two new `start methods <multiprocessing-start-methods>`{.interpreted-text role="ref"}, `spawn` and `forkserver`, have been added for starting processes using `multiprocessing`{.interpreted-text role="mod"}. These make the mixing of processes with threads more robust, and the `spawn` method matches the semantics that multiprocessing has always used on Windows. New function `~multiprocessing.get_all_start_methods`{.interpreted-text role="func"} reports all start methods available on the platform, `~multiprocessing.get_start_method`{.interpreted-text role="func"} reports the current start method, and `~multiprocessing.set_start_method`{.interpreted-text role="func"} sets the start method. (Contributed by Richard Oudkerk in `8713`{.interpreted-text role="issue"}.)
:::

`multiprocessing`{.interpreted-text role="mod"} also now has the concept of a `context`, which determines how child processes are created. New function `~multiprocessing.get_context`{.interpreted-text role="func"} returns a context that uses a specified start method. It has the same API as the `multiprocessing`{.interpreted-text role="mod"} module itself, so you can use it to create `~multiprocessing.pool.Pool`{.interpreted-text role="class"}s and other objects that will operate within that context. This allows a framework and an application or different parts of the same application to use multiprocessing without interfering with each other. (Contributed by Richard Oudkerk in `18999`{.interpreted-text role="issue"}.)

Except when using the old *fork* start method, child processes no longer inherit unneeded handles/file descriptors from their parents (part of `8713`{.interpreted-text role="issue"}).

`multiprocessing`{.interpreted-text role="mod"} now relies on `runpy`{.interpreted-text role="mod"} (which implements the `-m` switch) to initialise `__main__` appropriately in child processes when using the `spawn` or `forkserver` start methods. This resolves some edge cases where combining multiprocessing, the `-m` command line switch, and explicit relative imports could cause obscure failures in child processes. (Contributed by Nick Coghlan in `19946`{.interpreted-text role="issue"}.)

### operator

New function `~operator.length_hint`{.interpreted-text role="func"} provides an implementation of the specification for how the `~object.__length_hint__`{.interpreted-text role="meth"} special method should be used, as part of the `424`{.interpreted-text role="pep"} formal specification of this language feature. (Contributed by Armin Ronacher in `16148`{.interpreted-text role="issue"}.)

There is now a pure-python version of the `operator`{.interpreted-text role="mod"} module available for reference and for use by alternate implementations of Python. (Contributed by Zachary Ware in `16694`{.interpreted-text role="issue"}.)

### os

There are new functions to get and set the `inheritable flag
<fd_inheritance>`{.interpreted-text role="ref"} of a file descriptor (`os.get_inheritable`{.interpreted-text role="func"}, `os.set_inheritable`{.interpreted-text role="func"}) or a Windows handle (`os.get_handle_inheritable`{.interpreted-text role="func"}, `os.set_handle_inheritable`{.interpreted-text role="func"}).

New function `~os.cpu_count`{.interpreted-text role="func"} reports the number of CPUs available on the platform on which Python is running (or `None` if the count can\'t be determined). The `multiprocessing.cpu_count`{.interpreted-text role="func"} function is now implemented in terms of this function). (Contributed by Trent Nelson, Yogesh Chaudhari, Victor Stinner, and Charles-François Natali in `17914`{.interpreted-text role="issue"}.)

`os.path.samestat`{.interpreted-text role="func"} is now available on the Windows platform (and the `os.path.samefile`{.interpreted-text role="func"} implementation is now shared between Unix and Windows). (Contributed by Brian Curtin in `11939`{.interpreted-text role="issue"}.)

`os.path.ismount`{.interpreted-text role="func"} now recognizes volumes mounted below a drive root on Windows. (Contributed by Tim Golden in `9035`{.interpreted-text role="issue"}.)

`os.open`{.interpreted-text role="func"} supports two new flags on platforms that provide them, `~os.O_PATH`{.interpreted-text role="const"} (un-opened file descriptor), and `~os.O_TMPFILE`{.interpreted-text role="const"} (unnamed temporary file; as of 3.4.0 release available only on Linux systems with a kernel version of 3.11 or newer that have uapi headers). (Contributed by Christian Heimes in `18673`{.interpreted-text role="issue"} and Benjamin Peterson, respectively.)

### pdb

`pdb`{.interpreted-text role="mod"} has been enhanced to handle generators, `yield`{.interpreted-text role="keyword"}, and `yield from` in a more useful fashion. This is especially helpful when debugging `asyncio`{.interpreted-text role="mod"} based programs. (Contributed by Andrew Svetlov and Xavier de Gaye in `16596`{.interpreted-text role="issue"}.)

The `print` command has been removed from `pdb`{.interpreted-text role="mod"}, restoring access to the Python `print`{.interpreted-text role="func"} function from the pdb command line. Python2\'s `pdb` did not have a `print` command; instead, entering `print` executed the `print` statement. In Python3 `print` was mistakenly made an alias for the pdb `p`{.interpreted-text role="pdbcmd"} command. `p`, however, prints the `repr` of its argument, not the `str` like the Python2 `print` command did. Worse, the Python3 `pdb print` command shadowed the Python3 `print` function, making it inaccessible at the `pdb` prompt. (Contributed by Connor Osborn in `18764`{.interpreted-text role="issue"}.)

### pickle {#whatsnew-protocol-4}

`pickle`{.interpreted-text role="mod"} now supports (but does not use by default) a new pickle protocol, protocol 4. This new protocol addresses a number of issues that were present in previous protocols, such as the serialization of nested classes, very large strings and containers, and classes whose `~object.__new__`{.interpreted-text role="meth"} method takes keyword-only arguments. It also provides some efficiency improvements.

::: seealso

`3154`{.interpreted-text role="pep"} \-- Pickle protocol 4

:   PEP written by Antoine Pitrou and implemented by Alexandre Vassalotti.
:::

### plistlib

`plistlib`{.interpreted-text role="mod"} now has an API that is similar to the standard pattern for stdlib serialization protocols, with new `~plistlib.load`{.interpreted-text role="func"}, `~plistlib.dump`{.interpreted-text role="func"}, `~plistlib.loads`{.interpreted-text role="func"}, and `~plistlib.dumps`{.interpreted-text role="func"} functions. (The older API is now deprecated.) In addition to the already supported XML plist format (`~plistlib.FMT_XML`{.interpreted-text role="const"}), it also now supports the binary plist format (`~plistlib.FMT_BINARY`{.interpreted-text role="const"}). (Contributed by Ronald Oussoren and others in `14455`{.interpreted-text role="issue"}.)

### poplib

Two new methods have been added to `poplib`{.interpreted-text role="mod"}: `~poplib.POP3.capa`{.interpreted-text role="meth"}, which returns the list of capabilities advertised by the POP server, and `~poplib.POP3.stls`{.interpreted-text role="meth"}, which switches a clear-text POP3 session into an encrypted POP3 session if the POP server supports it. (Contributed by Lorenzo Catucci in `4473`{.interpreted-text role="issue"}.)

### pprint

The `pprint`{.interpreted-text role="mod"} module\'s `~pprint.PrettyPrinter`{.interpreted-text role="class"} class and its `~pprint.pformat`{.interpreted-text role="func"}, and `~pprint.pprint`{.interpreted-text role="func"} functions have a new option, *compact*, that controls how the output is formatted. Currently setting *compact* to `True` means that sequences will be printed with as many sequence elements as will fit within *width* on each (indented) line. (Contributed by Serhiy Storchaka in `19132`{.interpreted-text role="issue"}.)

Long strings are now wrapped using Python\'s normal line continuation syntax. (Contributed by Antoine Pitrou in `17150`{.interpreted-text role="issue"}.)

### pty

`pty.spawn`{.interpreted-text role="func"} now returns the status value from `os.waitpid`{.interpreted-text role="func"} on the child process, instead of `None`. (Contributed by Gregory P. Smith.)

### pydoc

The `pydoc`{.interpreted-text role="mod"} module is now based directly on the `inspect.signature`{.interpreted-text role="func"} introspection API, allowing it to provide signature information for a wider variety of callable objects. This change also means that `__wrapped__` attributes are now taken into account when displaying help information. (Contributed by Larry Hastings in `19674`{.interpreted-text role="issue"}.)

The `pydoc`{.interpreted-text role="mod"} module no longer displays the `self` parameter for already bound methods. Instead, it aims to always display the exact current signature of the supplied callable. (Contributed by Larry Hastings in `20710`{.interpreted-text role="issue"}.)

In addition to the changes that have been made to `pydoc`{.interpreted-text role="mod"} directly, its handling of custom `__dir__` methods and various descriptor behaviours has also been improved substantially by the underlying changes in the `inspect`{.interpreted-text role="mod"} module.

As the `help`{.interpreted-text role="func"} builtin is based on `pydoc`{.interpreted-text role="mod"}, the above changes also affect the behaviour of `help`{.interpreted-text role="func"}.

### re

New `~re.fullmatch`{.interpreted-text role="func"} function and `.Pattern.fullmatch`{.interpreted-text role="meth"} method anchor the pattern at both ends of the string to match. This provides a way to be explicit about the goal of the match, which avoids a class of subtle bugs where `$` characters get lost during code changes or the addition of alternatives to an existing regular expression. (Contributed by Matthew Barnett in `16203`{.interpreted-text role="issue"}.)

The repr of `regex objects <re-objects>`{.interpreted-text role="ref"} now includes the pattern and the flags; the repr of `match objects <match-objects>`{.interpreted-text role="ref"} now includes the start, end, and the part of the string that matched. (Contributed by Hugo Lopes Tavares and Serhiy Storchaka in `13592`{.interpreted-text role="issue"} and `17087`{.interpreted-text role="issue"}.)

### resource

New `~resource.prlimit`{.interpreted-text role="func"} function, available on Linux platforms with a kernel version of 2.6.36 or later and glibc of 2.13 or later, provides the ability to query or set the resource limits for processes other than the one making the call. (Contributed by Christian Heimes in `16595`{.interpreted-text role="issue"}.)

On Linux kernel version 2.6.36 or later, there are also some new Linux specific constants: `~resource.RLIMIT_MSGQUEUE`{.interpreted-text role="const"}, `~resource.RLIMIT_NICE`{.interpreted-text role="const"}, `~resource.RLIMIT_RTPRIO`{.interpreted-text role="const"}, `~resource.RLIMIT_RTTIME`{.interpreted-text role="const"}, and `~resource.RLIMIT_SIGPENDING`{.interpreted-text role="const"}. (Contributed by Christian Heimes in `19324`{.interpreted-text role="issue"}.)

On FreeBSD version 9 and later, there some new FreeBSD specific constants: `~resource.RLIMIT_SBSIZE`{.interpreted-text role="const"}, `~resource.RLIMIT_SWAP`{.interpreted-text role="const"}, and `~resource.RLIMIT_NPTS`{.interpreted-text role="const"}. (Contributed by Claudiu Popa in `19343`{.interpreted-text role="issue"}.)

### select

`~select.epoll`{.interpreted-text role="class"} objects now support the context management protocol. When used in a `with`{.interpreted-text role="keyword"} statement, the `~select.epoll.close`{.interpreted-text role="meth"} method will be called automatically at the end of the block. (Contributed by Serhiy Storchaka in `16488`{.interpreted-text role="issue"}.)

`~select.devpoll`{.interpreted-text role="class"} objects now have `~select.devpoll.fileno`{.interpreted-text role="meth"} and `~select.devpoll.close`{.interpreted-text role="meth"} methods, as well as a new attribute `~select.devpoll.closed`{.interpreted-text role="attr"}. (Contributed by Victor Stinner in `18794`{.interpreted-text role="issue"}.)

### shelve

`~shelve.Shelf`{.interpreted-text role="class"} instances may now be used in `with`{.interpreted-text role="keyword"} statements, and will be automatically closed at the end of the `!with`{.interpreted-text role="keyword"} block. (Contributed by Filip Gruszczyński in `13896`{.interpreted-text role="issue"}.)

### shutil

`~shutil.copyfile`{.interpreted-text role="func"} now raises a specific `~shutil.Error`{.interpreted-text role="exc"} subclass, `~shutil.SameFileError`{.interpreted-text role="exc"}, when the source and destination are the same file, which allows an application to take appropriate action on this specific error. (Contributed by Atsuo Ishimoto and Hynek Schlawack in `1492704`{.interpreted-text role="issue"}.)

### smtpd

The `!SMTPServer`{.interpreted-text role="class"} and `!SMTPChannel`{.interpreted-text role="class"} classes now accept a *map* keyword argument which, if specified, is passed in to `!asynchat.async_chat`{.interpreted-text role="class"} as its *map* argument. This allows an application to avoid affecting the global socket map. (Contributed by Vinay Sajip in `11959`{.interpreted-text role="issue"}.)

### smtplib

`~smtplib.SMTPException`{.interpreted-text role="exc"} is now a subclass of `OSError`{.interpreted-text role="exc"}, which allows both socket level errors and SMTP protocol level errors to be caught in one try/except statement by code that only cares whether or not an error occurred. (Contributed by Ned Jackson Lovely in `2118`{.interpreted-text role="issue"}.)

### socket

The socket module now supports the `~socket.CAN_BCM`{.interpreted-text role="const"} protocol on platforms that support it. (Contributed by Brian Thorne in `15359`{.interpreted-text role="issue"}.)

Socket objects have new methods to get or set their `inheritable flag
<fd_inheritance>`{.interpreted-text role="ref"}, `~socket.socket.get_inheritable`{.interpreted-text role="meth"} and `~socket.socket.set_inheritable`{.interpreted-text role="meth"}.

The `socket.AF_*` and `socket.SOCK_*` constants are now enumeration values using the new `enum`{.interpreted-text role="mod"} module. This allows meaningful names to be printed during debugging, instead of integer \"magic numbers\".

The `~socket.AF_LINK`{.interpreted-text role="const"} constant is now available on BSD and OSX.

`~socket.inet_pton`{.interpreted-text role="func"} and `~socket.inet_ntop`{.interpreted-text role="func"} are now supported on Windows. (Contributed by Atsuo Ishimoto in `7171`{.interpreted-text role="issue"}.)

### sqlite3

A new boolean parameter to the `~sqlite3.connect`{.interpreted-text role="func"} function, *uri*, can be used to indicate that the *database* parameter is a `uri` (see the [SQLite URI documentation](https://www.sqlite.org/uri.html)). (Contributed by poq in `13773`{.interpreted-text role="issue"}.)

### ssl

::: {#whatsnew-tls-11-12}
`~ssl.PROTOCOL_TLSv1_1`{.interpreted-text role="data"} and `~ssl.PROTOCOL_TLSv1_2`{.interpreted-text role="data"} (TLSv1.1 and TLSv1.2 support) have been added; support for these protocols is only available if Python is linked with OpenSSL 1.0.1 or later. (Contributed by Michele Orrù and Antoine Pitrou in `16692`{.interpreted-text role="issue"}.)
:::

::: {#whatsnew34-sslcontext}
New function `~ssl.create_default_context`{.interpreted-text role="func"} provides a standard way to obtain an `~ssl.SSLContext`{.interpreted-text role="class"} whose settings are intended to be a reasonable balance between compatibility and security. These settings are more stringent than the defaults provided by the `~ssl.SSLContext`{.interpreted-text role="class"} constructor, and may be adjusted in the future, without prior deprecation, if best-practice security requirements change. The new recommended best practice for using stdlib libraries that support SSL is to use `~ssl.create_default_context`{.interpreted-text role="func"} to obtain an `~ssl.SSLContext`{.interpreted-text role="class"} object, modify it if needed, and then pass it as the *context* argument of the appropriate stdlib API. (Contributed by Christian Heimes in `19689`{.interpreted-text role="issue"}.)
:::

`~ssl.SSLContext`{.interpreted-text role="class"} method `~ssl.SSLContext.load_verify_locations`{.interpreted-text role="meth"} accepts a new optional argument *cadata*, which can be used to provide PEM or DER encoded certificates directly via strings or bytes, respectively. (Contributed by Christian Heimes in `18138`{.interpreted-text role="issue"}.)

New function `~ssl.get_default_verify_paths`{.interpreted-text role="func"} returns a named tuple of the paths and environment variables that the `~ssl.SSLContext.set_default_verify_paths`{.interpreted-text role="meth"} method uses to set OpenSSL\'s default `cafile` and `capath`. This can be an aid in debugging default verification issues. (Contributed by Christian Heimes in `18143`{.interpreted-text role="issue"}.)

`~ssl.SSLContext`{.interpreted-text role="class"} has a new method, `~ssl.SSLContext.cert_store_stats`{.interpreted-text role="meth"}, that reports the number of loaded `X.509` certs, `X.509 CA` certs, and certificate revocation lists (`crl`s), as well as a `~ssl.SSLContext.get_ca_certs`{.interpreted-text role="meth"} method that returns a list of the loaded `CA` certificates. (Contributed by Christian Heimes in `18147`{.interpreted-text role="issue"}.)

If OpenSSL 0.9.8 or later is available, `~ssl.SSLContext`{.interpreted-text role="class"} has a new attribute `~ssl.SSLContext.verify_flags`{.interpreted-text role="attr"} that can be used to control the certificate verification process by setting it to some combination of the new constants `~ssl.VERIFY_DEFAULT`{.interpreted-text role="const"}, `~ssl.VERIFY_CRL_CHECK_LEAF`{.interpreted-text role="const"}, `~ssl.VERIFY_CRL_CHECK_CHAIN`{.interpreted-text role="const"}, or `~ssl.VERIFY_X509_STRICT`{.interpreted-text role="const"}. OpenSSL does not do any CRL verification by default. (Contributed by Christien Heimes in `8813`{.interpreted-text role="issue"}.)

New `~ssl.SSLContext`{.interpreted-text role="class"} method `~ssl.SSLContext.load_default_certs`{.interpreted-text role="meth"} loads a set of default \"certificate authority\" (CA) certificates from default locations, which vary according to the platform. It can be used to load both TLS web server authentication certificates (`purpose=``~ssl.Purpose.SERVER_AUTH`{.interpreted-text role="data"}) for a client to use to verify a server, and certificates for a server to use in verifying client certificates (`purpose=``~ssl.Purpose.CLIENT_AUTH`{.interpreted-text role="data"}). (Contributed by Christian Heimes in `19292`{.interpreted-text role="issue"}.)

::: {#whatsnew34-win-cert-store}
Two new windows-only functions, `~ssl.enum_certificates`{.interpreted-text role="func"} and `~ssl.enum_crls`{.interpreted-text role="func"} provide the ability to retrieve certificates, certificate information, and CRLs from the Windows cert store. (Contributed by Christian Heimes in `17134`{.interpreted-text role="issue"}.)
:::

::: {#whatsnew34-sni}
Support for server-side SNI (Server Name Indication) using the new `ssl.SSLContext.set_servername_callback`{.interpreted-text role="meth"} method. (Contributed by Daniel Black in `8109`{.interpreted-text role="issue"}.)
:::

The dictionary returned by `.SSLSocket.getpeercert`{.interpreted-text role="meth"} contains additional `X509v3` extension items: `crlDistributionPoints`, `calIssuers`, and `OCSP` URIs. (Contributed by Christian Heimes in `18379`{.interpreted-text role="issue"}.)

### stat

The `stat`{.interpreted-text role="mod"} module is now backed by a C implementation in `!_stat`{.interpreted-text role="mod"}. A C implementation is required as most of the values aren\'t standardized and are platform-dependent. (Contributed by Christian Heimes in `11016`{.interpreted-text role="issue"}.)

The module supports new `~stat.ST_MODE`{.interpreted-text role="mod"} flags, `~stat.S_IFDOOR`{.interpreted-text role="mod"}, `~stat.S_IFPORT`{.interpreted-text role="const"}, and `~stat.S_IFWHT`{.interpreted-text role="const"}. (Contributed by Christian Hiemes in `11016`{.interpreted-text role="issue"}.)

### struct

New function `~struct.iter_unpack`{.interpreted-text role="mod"} and a new `struct.Struct.iter_unpack`{.interpreted-text role="meth"} method on compiled formats provide streamed unpacking of a buffer containing repeated instances of a given format of data. (Contributed by Antoine Pitrou in `17804`{.interpreted-text role="issue"}.)

### subprocess

`~subprocess.check_output`{.interpreted-text role="func"} now accepts an *input* argument that can be used to provide the contents of `stdin` for the command that is run. (Contributed by Zack Weinberg in `16624`{.interpreted-text role="issue"}.)

`~subprocess.getoutput`{.interpreted-text role="func"} and `~subprocess.getstatusoutput`{.interpreted-text role="func"} now work on Windows. This change was actually inadvertently made in 3.3.4. (Contributed by Tim Golden in `10197`{.interpreted-text role="issue"}.)

### sunau

The `!getparams`{.interpreted-text role="meth"} method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in `18901`{.interpreted-text role="issue"}.)

`!sunau.open`{.interpreted-text role="meth"} now supports the context management protocol: when used in a `with`{.interpreted-text role="keyword"} block, the `close` method of the returned object will be called automatically at the end of the block. (Contributed by Serhiy Storchaka in `18878`{.interpreted-text role="issue"}.)

`!AU_write.setsampwidth`{.interpreted-text role="meth"} now supports 24 bit samples, thus adding support for writing 24 sample using the module. (Contributed by Serhiy Storchaka in `19261`{.interpreted-text role="issue"}.)

The `!writeframesraw`{.interpreted-text role="meth"} and `!writeframes`{.interpreted-text role="meth"} methods now accept any `bytes-like
object`{.interpreted-text role="term"}. (Contributed by Serhiy Storchaka in `8311`{.interpreted-text role="issue"}.)

### sys

New function `sys.getallocatedblocks`{.interpreted-text role="func"} returns the current number of blocks allocated by the interpreter. (In CPython with the default `--with-pymalloc` setting, this is allocations made through the `PyObject_Malloc`{.interpreted-text role="c:func"} API.) This can be useful for tracking memory leaks, especially if automated via a test suite. (Contributed by Antoine Pitrou in `13390`{.interpreted-text role="issue"}.)

When the Python interpreter starts in `interactive mode
<tut-interactive>`{.interpreted-text role="ref"}, it checks for an `~sys.__interactivehook__`{.interpreted-text role="data"} attribute on the `sys`{.interpreted-text role="mod"} module. If the attribute exists, its value is called with no arguments just before interactive mode is started. The check is made after the `PYTHONSTARTUP`{.interpreted-text role="envvar"} file is read, so it can be set there. The `site`{.interpreted-text role="mod"} module `sets it <rlcompleter-config>`{.interpreted-text role="ref"} to a function that enables tab completion and history saving (in `~/.python-history`{.interpreted-text role="file"}) if the platform supports `readline`{.interpreted-text role="mod"}. If you do not want this (new) behavior, you can override it in `PYTHONSTARTUP`{.interpreted-text role="envvar"}, `sitecustomize`{.interpreted-text role="mod"}, or `usercustomize`{.interpreted-text role="mod"} by deleting this attribute from `sys`{.interpreted-text role="mod"} (or setting it to some other callable). (Contributed by Éric Araujo and Antoine Pitrou in `5845`{.interpreted-text role="issue"}.)

### tarfile

The `tarfile`{.interpreted-text role="mod"} module now supports a simple `tarfile-commandline`{.interpreted-text role="ref"} when called as a script directly or via `-m`. This can be used to create and extract tarfile archives. (Contributed by Berker Peksag in `13477`{.interpreted-text role="issue"}.)

### textwrap

The `~textwrap.TextWrapper`{.interpreted-text role="class"} class has two new attributes/constructor arguments: `~textwrap.TextWrapper.max_lines`{.interpreted-text role="attr"}, which limits the number of lines in the output, and `~textwrap.TextWrapper.placeholder`{.interpreted-text role="attr"}, which is a string that will appear at the end of the output if it has been truncated because of *max_lines*. Building on these capabilities, a new convenience function `~textwrap.shorten`{.interpreted-text role="func"} collapses all of the whitespace in the input to single spaces and produces a single line of a given *width* that ends with the *placeholder* (by default, `[...]`). (Contributed by Antoine Pitrou and Serhiy Storchaka in `18585`{.interpreted-text role="issue"} and `18725`{.interpreted-text role="issue"}.)

### threading

The `~threading.Thread`{.interpreted-text role="class"} object representing the main thread can be obtained from the new `~threading.main_thread`{.interpreted-text role="func"} function. In normal conditions this will be the thread from which the Python interpreter was started. (Contributed by Andrew Svetlov in `18882`{.interpreted-text role="issue"}.)

### traceback

A new `traceback.clear_frames`{.interpreted-text role="func"} function takes a traceback object and clears the local variables in all of the frames it references, reducing the amount of memory consumed. (Contributed by Andrew Kuchling in `1565525`{.interpreted-text role="issue"}.)

### types

A new `~types.DynamicClassAttribute`{.interpreted-text role="func"} descriptor provides a way to define an attribute that acts normally when looked up through an instance object, but which is routed to the *class* `__getattr__` when looked up through the class. This allows one to have properties active on a class, and have virtual attributes on the class with the same name (see `enum`{.interpreted-text role="mod"} for an example). (Contributed by Ethan Furman in `19030`{.interpreted-text role="issue"}.)

### urllib

`urllib.request`{.interpreted-text role="mod"} now supports `data:` URLs via the `~urllib.request.DataHandler`{.interpreted-text role="class"} class. (Contributed by Mathias Panzenböck in `16423`{.interpreted-text role="issue"}.)

The http method that will be used by a `~urllib.request.Request`{.interpreted-text role="class"} class can now be specified by setting a `~urllib.request.Request.method`{.interpreted-text role="class"} class attribute on the subclass. (Contributed by Jason R Coombs in `18978`{.interpreted-text role="issue"}.)

`~urllib.request.Request`{.interpreted-text role="class"} objects are now reusable: if the `~urllib.request.Request.full_url`{.interpreted-text role="attr"} or `~urllib.request.Request.data`{.interpreted-text role="attr"} attributes are modified, all relevant internal properties are updated. This means, for example, that it is now possible to use the same `~urllib.request.Request`{.interpreted-text role="class"} object in more than one `.OpenerDirector.open`{.interpreted-text role="meth"} call with different *data* arguments, or to modify a `~urllib.request.Request`{.interpreted-text role="class"}\'s `url` rather than recomputing it from scratch. There is also a new `~urllib.request.Request.remove_header`{.interpreted-text role="meth"} method that can be used to remove headers from a `~urllib.request.Request`{.interpreted-text role="class"}. (Contributed by Alexey Kachayev in `16464`{.interpreted-text role="issue"}, Daniel Wozniak in `17485`{.interpreted-text role="issue"}, and Damien Brecht and Senthil Kumaran in `17272`{.interpreted-text role="issue"}.)

`~urllib.error.HTTPError`{.interpreted-text role="class"} objects now have a `~urllib.error.HTTPError.headers`{.interpreted-text role="attr"} attribute that provides access to the HTTP response headers associated with the error. (Contributed by Berker Peksag in `15701`{.interpreted-text role="issue"}.)

### unittest

The `~unittest.TestCase`{.interpreted-text role="class"} class has a new method, `~unittest.TestCase.subTest`{.interpreted-text role="meth"}, that produces a context manager whose `with`{.interpreted-text role="keyword"} block becomes a \"sub-test\". This context manager allows a test method to dynamically generate subtests by, say, calling the `subTest` context manager inside a loop. A single test method can thereby produce an indefinite number of separately identified and separately counted tests, all of which will run even if one or more of them fail. For example:

    class NumbersTest(unittest.TestCase):
        def test_even(self):
            for i in range(6):
                with self.subTest(i=i):
                    self.assertEqual(i % 2, 0)

will result in six subtests, each identified in the unittest verbose output with a label consisting of the variable name `i` and a particular value for that variable (`i=0`, `i=1`, etc). See `subtests`{.interpreted-text role="ref"} for the full version of this example. (Contributed by Antoine Pitrou in `16997`{.interpreted-text role="issue"}.)

`unittest.main`{.interpreted-text role="func"} now accepts an iterable of test names for *defaultTest*, where previously it only accepted a single test name as a string. (Contributed by Jyrki Pulliainen in `15132`{.interpreted-text role="issue"}.)

If `~unittest.SkipTest`{.interpreted-text role="class"} is raised during test discovery (that is, at the module level in the test file), it is now reported as a skip instead of an error. (Contributed by Zach Ware in `16935`{.interpreted-text role="issue"}.)

`~unittest.TestLoader.discover`{.interpreted-text role="meth"} now sorts the discovered files to provide consistent test ordering. (Contributed by Martin Melin and Jeff Ramnani in `16709`{.interpreted-text role="issue"}.)

`~unittest.TestSuite`{.interpreted-text role="class"} now drops references to tests as soon as the test has been run, if the test is successful. On Python interpreters that do garbage collection, this allows the tests to be garbage collected if nothing else is holding a reference to the test. It is possible to override this behavior by creating a `~unittest.TestSuite`{.interpreted-text role="class"} subclass that defines a custom `_removeTestAtIndex` method. (Contributed by Tom Wardill, Matt McClure, and Andrew Svetlov in `11798`{.interpreted-text role="issue"}.)

A new test assertion context-manager, `~unittest.TestCase.assertLogs`{.interpreted-text role="meth"}, will ensure that a given block of code emits a log message using the `logging`{.interpreted-text role="mod"} module. By default the message can come from any logger and have a priority of `INFO` or higher, but both the logger name and an alternative minimum logging level may be specified. The object returned by the context manager can be queried for the `~logging.LogRecord`{.interpreted-text role="class"}s and/or formatted messages that were logged. (Contributed by Antoine Pitrou in `18937`{.interpreted-text role="issue"}.)

Test discovery now works with namespace packages (Contributed by Claudiu Popa in `17457`{.interpreted-text role="issue"}.)

`unittest.mock`{.interpreted-text role="mod"} objects now inspect their specification signatures when matching calls, which means an argument can now be matched by either position or name, instead of only by position. (Contributed by Antoine Pitrou in `17015`{.interpreted-text role="issue"}.)

`~unittest.mock.mock_open`{.interpreted-text role="func"} objects now have `readline` and `readlines` methods. (Contributed by Toshio Kuratomi in `17467`{.interpreted-text role="issue"}.)

### venv

`venv`{.interpreted-text role="mod"} now includes activation scripts for the `csh` and `fish` shells. (Contributed by Andrew Svetlov in `15417`{.interpreted-text role="issue"}.)

`~venv.EnvBuilder`{.interpreted-text role="class"} and the `~venv.create`{.interpreted-text role="func"} convenience function take a new keyword argument *with_pip*, which defaults to `False`, that controls whether or not `~venv.EnvBuilder`{.interpreted-text role="class"} ensures that `pip` is installed in the virtual environment. (Contributed by Nick Coghlan in `19552`{.interpreted-text role="issue"} as part of the `453`{.interpreted-text role="pep"} implementation.)

### wave

The `~wave.Wave_read.getparams`{.interpreted-text role="meth"} method now returns a namedtuple rather than a plain tuple. (Contributed by Claudiu Popa in `17487`{.interpreted-text role="issue"}.)

`wave.open`{.interpreted-text role="meth"} now supports the context management protocol. (Contributed by Claudiu Popa in `17616`{.interpreted-text role="issue"}.)

`wave`{.interpreted-text role="mod"} can now `write output to unseekable files
<wave-write-objects>`{.interpreted-text role="ref"}. (Contributed by David Jones, Guilherme Polo, and Serhiy Storchaka in `5202`{.interpreted-text role="issue"}.)

The `~wave.Wave_write.writeframesraw`{.interpreted-text role="meth"} and `~wave.Wave_write.writeframes`{.interpreted-text role="meth"} methods now accept any `bytes-like
object`{.interpreted-text role="term"}. (Contributed by Serhiy Storchaka in `8311`{.interpreted-text role="issue"}.)

### weakref

New `~weakref.WeakMethod`{.interpreted-text role="class"} class simulates weak references to bound methods. (Contributed by Antoine Pitrou in `14631`{.interpreted-text role="issue"}.)

New `~weakref.finalize`{.interpreted-text role="class"} class makes it possible to register a callback to be invoked when an object is garbage collected, without needing to carefully manage the lifecycle of the weak reference itself. (Contributed by Richard Oudkerk in `15528`{.interpreted-text role="issue"}.)

The callback, if any, associated with a `~weakref.ref`{.interpreted-text role="class"} is now exposed via the `~weakref.ref.__callback__`{.interpreted-text role="attr"} attribute. (Contributed by Mark Dickinson in `17643`{.interpreted-text role="issue"}.)

### xml.etree

A new parser, `~xml.etree.ElementTree.XMLPullParser`{.interpreted-text role="class"}, allows a non-blocking applications to parse XML documents. An example can be seen at `elementtree-pull-parsing`{.interpreted-text role="ref"}. (Contributed by Antoine Pitrou in `17741`{.interpreted-text role="issue"}.)

The `xml.etree.ElementTree`{.interpreted-text role="mod"} `~xml.etree.ElementTree.tostring`{.interpreted-text role="func"} and `~xml.etree.ElementTree.tostringlist`{.interpreted-text role="func"} functions, and the `~xml.etree.ElementTree.ElementTree`{.interpreted-text role="class"} `~xml.etree.ElementTree.ElementTree.write`{.interpreted-text role="meth"} method, now have a *short_empty_elements* `keyword-only parameter <keyword-only_parameter>`{.interpreted-text role="ref"} providing control over whether elements with no content are written in abbreviated (`<tag />`) or expanded (`<tag></tag>`) form. (Contributed by Ariel Poliak and Serhiy Storchaka in `14377`{.interpreted-text role="issue"}.)

### zipfile

The `~zipfile.PyZipFile.writepy`{.interpreted-text role="meth"} method of the `~zipfile.PyZipFile`{.interpreted-text role="class"} class has a new *filterfunc* option that can be used to control which directories and files are added to the archive. For example, this could be used to exclude test files from the archive. (Contributed by Christian Tismer in `19274`{.interpreted-text role="issue"}.)

The *allowZip64* parameter to `~zipfile.ZipFile`{.interpreted-text role="class"} and `~zipfile.PyZipFile`{.interpreted-text role="class"} is now `True` by default. (Contributed by William Mallard in `17201`{.interpreted-text role="issue"}.)

## CPython Implementation Changes

### PEP 445: Customization of CPython Memory Allocators {#whatsnew-pep-445}

`445`{.interpreted-text role="pep"} adds new C level interfaces to customize memory allocation in the CPython interpreter.

::: seealso

`445`{.interpreted-text role="pep"} \-- Add new APIs to customize Python memory allocators

:   PEP written and implemented by Victor Stinner.
:::

### PEP 442: Safe Object Finalization {#whatsnew-pep-442}

`442`{.interpreted-text role="pep"} removes the current limitations and quirks of object finalization in CPython. With it, objects with `~object.__del__`{.interpreted-text role="meth"} methods, as well as generators with `finally`{.interpreted-text role="keyword"} clauses, can be finalized when they are part of a reference cycle.

As part of this change, module globals are no longer forcibly set to `None`{.interpreted-text role="const"} during interpreter shutdown in most cases, instead relying on the normal operation of the cyclic garbage collector. This avoids a whole class of interpreter-shutdown-time errors, usually involving `__del__` methods, that have plagued Python since the cyclic GC was first introduced.

::: seealso

`442`{.interpreted-text role="pep"} \-- Safe object finalization

:   PEP written and implemented by Antoine Pitrou.
:::

### PEP 456: Secure and Interchangeable Hash Algorithm {#whatsnew-pep-456}

`456`{.interpreted-text role="pep"} follows up on earlier security fix work done on Python\'s hash algorithm to address certain DOS attacks to which public facing APIs backed by dictionary lookups may be subject. (See `14621`{.interpreted-text role="issue"} for the start of the current round of improvements.) The PEP unifies CPython\'s hash code to make it easier for a packager to substitute a different hash algorithm, and switches Python\'s default implementation to a SipHash implementation on platforms that have a 64 bit data type. Any performance differences in comparison with the older FNV algorithm are trivial.

The PEP adds additional fields to the `sys.hash_info`{.interpreted-text role="data"} named tuple to describe the hash algorithm in use by the currently executing binary. Otherwise, the PEP does not alter any existing CPython APIs.

### PEP 436: Argument Clinic {#whatsnew-pep-436}

\"Argument Clinic\" (`436`{.interpreted-text role="pep"}) is now part of the CPython build process and can be used to simplify the process of defining and maintaining accurate signatures for builtins and standard library extension modules implemented in C.

Some standard library extension modules have been converted to use Argument Clinic in Python 3.4, and `pydoc`{.interpreted-text role="mod"} and `inspect`{.interpreted-text role="mod"} have been updated accordingly.

It is expected that signature metadata for programmatic introspection will be added to additional callables implemented in C as part of Python 3.4 maintenance releases.

:::: note
::: title
Note
:::

The Argument Clinic PEP is not fully up to date with the state of the implementation. This has been deemed acceptable by the release manager and core development team in this case, as Argument Clinic will not be made available as a public API for third party use in Python 3.4.
::::

::: seealso

`436`{.interpreted-text role="pep"} \-- The Argument Clinic DSL

:   PEP written and implemented by Larry Hastings.
:::

### Other Build and C API Changes

- The new `PyType_GetSlot`{.interpreted-text role="c:func"} function has been added to the stable ABI, allowing retrieval of function pointers from named type slots when using the limited API. (Contributed by Martin von Löwis in `17162`{.interpreted-text role="issue"}.)
- The new `!Py_SetStandardStreamEncoding`{.interpreted-text role="c:func"} pre-initialization API allows applications embedding the CPython interpreter to reliably force a particular encoding and error handler for the standard streams. (Contributed by Bastien Montagne and Nick Coghlan in `16129`{.interpreted-text role="issue"}.)
- Most Python C APIs that don\'t mutate string arguments are now correctly marked as accepting `const char *` rather than `char *`. (Contributed by Serhiy Storchaka in `1772673`{.interpreted-text role="issue"}.)
- A new shell version of `python-config` can be used even when a python interpreter is not available (for example, in cross compilation scenarios).
- `PyUnicode_FromFormat`{.interpreted-text role="c:func"} now supports width and precision specifications for `%s`, `%A`, `%U`, `%V`, `%S`, and `%R`. (Contributed by Ysj Ray and Victor Stinner in `7330`{.interpreted-text role="issue"}.)
- New function `PyStructSequence_InitType2`{.interpreted-text role="c:func"} supplements the existing `PyStructSequence_InitType`{.interpreted-text role="c:func"} function. The difference is that it returns `0` on success and `-1` on failure.
- The CPython source can now be compiled using the address sanity checking features of recent versions of GCC and clang: the false alarms in the small object allocator have been silenced. (Contributed by Dhiru Kholia in `18596`{.interpreted-text role="issue"}.)
- The Windows build now uses [Address Space Layout Randomization](https://en.wikipedia.org/wiki/Address_space_layout_randomization) and [Data Execution Prevention](https://en.wikipedia.org/wiki/Data_Execution_Prevention). (Contributed by Christian Heimes in `16632`{.interpreted-text role="issue"}.)
- New function `PyObject_LengthHint`{.interpreted-text role="c:func"} is the C API equivalent of `operator.length_hint`{.interpreted-text role="func"}. (Contributed by Armin Ronacher in `16148`{.interpreted-text role="issue"}.)

### Other Improvements {#other-improvements-3.4}

::: {#whatsnew-isolated-mode}
- The `python <using-on-cmdline>`{.interpreted-text role="ref"} command has a new `option
  <using-on-misc-options>`{.interpreted-text role="ref"}, `-I`, which causes it to run in \"isolated mode\", which means that `sys.path`{.interpreted-text role="data"} contains neither the script\'s directory nor the user\'s `site-packages` directory, and all `!PYTHON*`{.interpreted-text role="envvar"} environment variables are ignored (it implies both `-s` and `-E`). Other restrictions may also be applied in the future, with the goal being to isolate the execution of a script from the user\'s environment. This is appropriate, for example, when Python is used to run a system script. On most POSIX systems it can and should be used in the `#!` line of system scripts. (Contributed by Christian Heimes in `16499`{.interpreted-text role="issue"}.)
- Tab-completion is now enabled by default in the interactive interpreter on systems that support `readline`{.interpreted-text role="mod"}. History is also enabled by default, and is written to (and read from) the file `~/.python-history`{.interpreted-text role="file"}. (Contributed by Antoine Pitrou and Éric Araujo in `5845`{.interpreted-text role="issue"}.)
- Invoking the Python interpreter with `--version` now outputs the version to standard output instead of standard error (`18338`{.interpreted-text role="issue"}). Similar changes were made to `argparse`{.interpreted-text role="mod"} (`18920`{.interpreted-text role="issue"}) and other modules that have script-like invocation capabilities (`18922`{.interpreted-text role="issue"}).
- The CPython Windows installer now adds `.py` to the `PATHEXT`{.interpreted-text role="envvar"} variable when extensions are registered, allowing users to run a python script at the windows command prompt by just typing its name without the `.py` extension. (Contributed by Paul Moore in `18569`{.interpreted-text role="issue"}.)
- A new `make` target [coverage-report](https://devguide.python.org/coverage/#measuring-coverage-of-c-code-with-gcov-and-lcov) will build python, run the test suite, and generate an HTML coverage report for the C codebase using `gcov` and [lcov](https://github.com/linux-test-project/lcov).
- The `-R` option to the `python regression test suite <regrtest>`{.interpreted-text role="ref"} now also checks for memory allocation leaks, using `sys.getallocatedblocks`{.interpreted-text role="func"}. (Contributed by Antoine Pitrou in `13390`{.interpreted-text role="issue"}.)
- `python -m` now works with namespace packages.
- The `stat`{.interpreted-text role="mod"} module is now implemented in C, which means it gets the values for its constants from the C header files, instead of having the values hard-coded in the python module as was previously the case.
- Loading multiple python modules from a single OS module (`.so`, `.dll`) now works correctly (previously it silently returned the first python module in the file). (Contributed by Václav Šmilauer in `16421`{.interpreted-text role="issue"}.)
- A new opcode, `!LOAD_CLASSDEREF`{.interpreted-text role="opcode"}, has been added to fix a bug in the loading of free variables in class bodies that could be triggered by certain uses of `__prepare__ <prepare>`{.interpreted-text role="ref"}. (Contributed by Benjamin Peterson in `17853`{.interpreted-text role="issue"}.)
- A number of MemoryError-related crashes were identified and fixed by Victor Stinner using his `445`{.interpreted-text role="pep"}-based `pyfailmalloc` tool (`18408`{.interpreted-text role="issue"}, `18520`{.interpreted-text role="issue"}).
- The `pyvenv` command now accepts a `--copies` option to use copies rather than symlinks even on systems where symlinks are the default. (Contributed by Vinay Sajip in `18807`{.interpreted-text role="issue"}.)
- The `pyvenv` command also accepts a `--without-pip` option to suppress the otherwise-automatic bootstrapping of pip into the virtual environment. (Contributed by Nick Coghlan in `19552`{.interpreted-text role="issue"} as part of the `453`{.interpreted-text role="pep"} implementation.)
- The encoding name is now optional in the value set for the `PYTHONIOENCODING`{.interpreted-text role="envvar"} environment variable. This makes it possible to set just the error handler, without changing the default encoding. (Contributed by Serhiy Storchaka in `18818`{.interpreted-text role="issue"}.)
- The `bz2`{.interpreted-text role="mod"}, `lzma`{.interpreted-text role="mod"}, and `gzip`{.interpreted-text role="mod"} module `open` functions now support `x` (exclusive creation) mode. (Contributed by Tim Heaney and Vajrasky Kok in `19201`{.interpreted-text role="issue"}, `19222`{.interpreted-text role="issue"}, and `19223`{.interpreted-text role="issue"}.)
:::

### Significant Optimizations

- The UTF-32 decoder is now 3x to 4x faster. (Contributed by Serhiy Storchaka in `14625`{.interpreted-text role="issue"}.)
- The cost of hash collisions for sets is now reduced. Each hash table probe now checks a series of consecutive, adjacent key/hash pairs before continuing to make random probes through the hash table. This exploits cache locality to make collision resolution less expensive. The collision resolution scheme can be described as a hybrid of linear probing and open addressing. The number of additional linear probes defaults to nine. This can be changed at compile-time by defining LINEAR_PROBES to be any value. Set LINEAR_PROBES=0 to turn-off linear probing entirely. (Contributed by Raymond Hettinger in `18771`{.interpreted-text role="issue"}.)
- The interpreter starts about 30% faster. A couple of measures lead to the speedup. The interpreter loads fewer modules on startup, e.g. the `re`{.interpreted-text role="mod"}, `collections`{.interpreted-text role="mod"} and `locale`{.interpreted-text role="mod"} modules and their dependencies are no longer imported by default. The marshal module has been improved to load compiled Python code faster. (Contributed by Antoine Pitrou, Christian Heimes and Victor Stinner in `19219`{.interpreted-text role="issue"}, `19218`{.interpreted-text role="issue"}, `19209`{.interpreted-text role="issue"}, `19205`{.interpreted-text role="issue"} and `9548`{.interpreted-text role="issue"}.)
- `bz2.BZ2File`{.interpreted-text role="class"} is now as fast or faster than the Python2 version for most cases. `lzma.LZMAFile`{.interpreted-text role="class"} has also been optimized. (Contributed by Serhiy Storchaka and Nadeem Vawda in `16034`{.interpreted-text role="issue"}.)
- `random.getrandbits`{.interpreted-text role="func"} is 20%-40% faster for small integers (the most common use case). (Contributed by Serhiy Storchaka in `16674`{.interpreted-text role="issue"}.)
- By taking advantage of the new storage format for strings, pickling of strings is now significantly faster. (Contributed by Victor Stinner and Antoine Pitrou in `15596`{.interpreted-text role="issue"}.)
- A performance issue in `!io.FileIO.readall`{.interpreted-text role="meth"} has been solved. This particularly affects Windows, and significantly speeds up the case of piping significant amounts of data through `subprocess`{.interpreted-text role="mod"}. (Contributed by Richard Oudkerk in `15758`{.interpreted-text role="issue"}.)
- `html.escape`{.interpreted-text role="func"} is now 10x faster. (Contributed by Matt Bryant in `18020`{.interpreted-text role="issue"}.)
- On Windows, the native `VirtualAlloc` is now used instead of the CRT `malloc` in `obmalloc`. Artificial benchmarks show about a 3% memory savings.
- `os.urandom`{.interpreted-text role="func"} now uses a lazily opened persistent file descriptor so as to avoid using many file descriptors when run in parallel from multiple threads. (Contributed by Antoine Pitrou in `18756`{.interpreted-text role="issue"}.)

## Deprecated {#deprecated-3.4}

This section covers various APIs and other features that have been deprecated in Python 3.4, and will be removed in Python 3.5 or later. In most (but not all) cases, using the deprecated APIs will produce a `DeprecationWarning`{.interpreted-text role="exc"} when the interpreter is run with deprecation warnings enabled (for example, by using `-Wd`).

### Deprecations in the Python API

- As mentioned in `whatsnew-pep-451`{.interpreted-text role="ref"}, a number of `importlib`{.interpreted-text role="mod"} methods and functions are deprecated: `!importlib.find_loader`{.interpreted-text role="meth"} is replaced by `importlib.util.find_spec`{.interpreted-text role="func"}; `!importlib.machinery.PathFinder.find_module`{.interpreted-text role="meth"} is replaced by `importlib.machinery.PathFinder.find_spec`{.interpreted-text role="meth"}; `!importlib.abc.MetaPathFinder.find_module`{.interpreted-text role="meth"} is replaced by `importlib.abc.MetaPathFinder.find_spec`{.interpreted-text role="meth"}; `!importlib.abc.PathEntryFinder.find_loader`{.interpreted-text role="meth"} and `!find_module`{.interpreted-text role="meth"} are replaced by `importlib.abc.PathEntryFinder.find_spec`{.interpreted-text role="meth"}; all of the `{xxx}Loader`{.interpreted-text role="samp"} ABC `load_module` methods (`importlib.abc.Loader.load_module`, `importlib.abc.InspectLoader.load_module`, `importlib.abc.FileLoader.load_module`, `importlib.abc.SourceLoader.load_module`) should no longer be implemented, instead loaders should implement an `exec_module` method (`importlib.abc.Loader.exec_module`{.interpreted-text role="meth"}, `importlib.abc.InspectLoader.exec_module`{.interpreted-text role="meth"} `importlib.abc.SourceLoader.exec_module`{.interpreted-text role="meth"}) and let the import system take care of the rest; and `!importlib.abc.Loader.module_repr`{.interpreted-text role="meth"}, `!importlib.util.module_for_loader`{.interpreted-text role="meth"}, `!importlib.util.set_loader`{.interpreted-text role="meth"}, and `!importlib.util.set_package`{.interpreted-text role="meth"} are no longer needed because their functions are now handled automatically by the import system.
- The `!imp`{.interpreted-text role="mod"} module is pending deprecation. To keep compatibility with Python 2/3 code bases, the module\'s removal is currently not scheduled.
- The `!formatter`{.interpreted-text role="mod"} module is pending deprecation and is slated for removal in Python 3.6.
- `MD5` as the default *digestmod* for the `hmac.new`{.interpreted-text role="func"} function is deprecated. Python 3.6 will require an explicit digest name or constructor as *digestmod* argument.
- The internal `Netrc` class in the `ftplib`{.interpreted-text role="mod"} module has been documented as deprecated in its docstring for quite some time. It now emits a `DeprecationWarning`{.interpreted-text role="exc"} and will be removed completely in Python 3.5.
- The undocumented *endtime* argument to `subprocess.Popen.wait`{.interpreted-text role="meth"} should not have been exposed and is hopefully not in use; it is deprecated and will mostly likely be removed in Python 3.5.
- The *strict* argument of `~html.parser.HTMLParser`{.interpreted-text role="class"} is deprecated.
- The `plistlib`{.interpreted-text role="mod"} `!readPlist`{.interpreted-text role="func"}, `!writePlist`{.interpreted-text role="func"}, `!readPlistFromBytes`{.interpreted-text role="func"}, and `!writePlistToBytes`{.interpreted-text role="func"} functions are deprecated in favor of the corresponding new functions `~plistlib.load`{.interpreted-text role="func"}, `~plistlib.dump`{.interpreted-text role="func"}, `~plistlib.loads`{.interpreted-text role="func"}, and `~plistlib.dumps`{.interpreted-text role="func"}. `!Data`{.interpreted-text role="func"} is deprecated in favor of just using the `bytes`{.interpreted-text role="class"} constructor.
- The `sysconfig`{.interpreted-text role="mod"} key `SO` is deprecated, it has been replaced by `EXT_SUFFIX`.
- The `U` mode accepted by various `open` functions is deprecated. In Python3 it does not do anything useful, and should be replaced by appropriate uses of `io.TextIOWrapper`{.interpreted-text role="class"} (if needed) and its *newline* argument.
- The *parser* argument of `xml.etree.ElementTree.iterparse`{.interpreted-text role="func"} has been deprecated, as has the *html* argument of `~xml.etree.ElementTree.XMLParser`{.interpreted-text role="func"}. To prepare for the removal of the latter, all arguments to `XMLParser` should be passed by keyword.

### Deprecated Features

- Running `idle`{.interpreted-text role="ref"} with the `-n` flag (no subprocess) is deprecated. However, the feature will not be removed until `18823`{.interpreted-text role="issue"} is resolved.
- The site module adding a \"site-python\" directory to sys.path, if it exists, is deprecated (`19375`{.interpreted-text role="issue"}).

## Removed

### Operating Systems No Longer Supported

Support for the following operating systems has been removed from the source and build tools:

- OS/2 (`16135`{.interpreted-text role="issue"}).
- Windows 2000 (changeset e52df05b496a).
- Windows systems where `COMSPEC` points to `command.com` (`14470`{.interpreted-text role="issue"}).
- VMS (`16136`{.interpreted-text role="issue"}).

### API and Feature Removals

The following obsolete and previously deprecated APIs and features have been removed:

- The unmaintained `Misc/TextMate` and `Misc/vim` directories have been removed (see the [devguide](https://devguide.python.org) for suggestions on what to use instead).
- The `SO` makefile macro is removed (it was replaced by the `SHLIB_SUFFIX` and `EXT_SUFFIX` macros) (`16754`{.interpreted-text role="issue"}).
- The `PyThreadState.tick_counter` field has been removed; its value has been meaningless since Python 3.2, when the \"new GIL\" was introduced (`19199`{.interpreted-text role="issue"}).
- `PyLoader` and `PyPycLoader` have been removed from `importlib`{.interpreted-text role="mod"}. (Contributed by Taras Lyapun in `15641`{.interpreted-text role="issue"}.)
- The *strict* argument to `~http.client.HTTPConnection`{.interpreted-text role="class"} and `~http.client.HTTPSConnection`{.interpreted-text role="class"} has been removed. HTTP 0.9-style \"Simple Responses\" are no longer supported.
- The deprecated `urllib.request.Request`{.interpreted-text role="mod"} getter and setter methods `add_data`, `has_data`, `get_data`, `get_type`, `get_host`, `get_selector`, `set_proxy`, `get_origin_req_host`, and `is_unverifiable` have been removed (use direct attribute access instead).
- Support for loading the deprecated `TYPE_INT64` has been removed from `marshal`{.interpreted-text role="mod"}. (Contributed by Dan Riti in `15480`{.interpreted-text role="issue"}.)
- `inspect.Signature`{.interpreted-text role="class"}: positional-only parameters are now required to have a valid name.
- `object.__format__`{.interpreted-text role="meth"} no longer accepts non-empty format strings, it now raises a `TypeError`{.interpreted-text role="exc"} instead. Using a non-empty string has been deprecated since Python 3.2. This change has been made to prevent a situation where previously working (but incorrect) code would start failing if an object gained a \_\_format\_\_ method, which means that your code may now raise a `TypeError`{.interpreted-text role="exc"} if you are using an `'s'` format code with objects that do not have a \_\_format\_\_ method that handles it. See `7994`{.interpreted-text role="issue"} for background.
- `!difflib.SequenceMatcher.isbjunk`{.interpreted-text role="meth"} and `!difflib.SequenceMatcher.isbpopular`{.interpreted-text role="meth"} were deprecated in 3.2, and have now been removed: use `x in sm.bjunk` and `x in sm.bpopular`, where *sm* is a `~difflib.SequenceMatcher`{.interpreted-text role="class"} object (`13248`{.interpreted-text role="issue"}).

### Code Cleanups

- The unused and undocumented internal `Scanner` class has been removed from the `pydoc`{.interpreted-text role="mod"} module.
- The private and effectively unused `_gestalt` module has been removed, along with the private `platform`{.interpreted-text role="mod"} functions `_mac_ver_lookup`, `_mac_ver_gstalt`, and `_bcd2str`, which would only have ever been called on badly broken OSX systems (see `18393`{.interpreted-text role="issue"}).
- The hardcoded copies of certain `stat`{.interpreted-text role="mod"} constants that were included in the `tarfile`{.interpreted-text role="mod"} module namespace have been removed.

## Porting to Python 3.4

This section lists previously described changes and other bugfixes that may require changes to your code.

### Changes in \'python\' Command Behavior

- In a posix shell, setting the `PATH`{.interpreted-text role="envvar"} environment variable to an empty value is equivalent to not setting it at all. However, setting `PYTHONPATH`{.interpreted-text role="envvar"} to an empty value was *not* equivalent to not setting it at all: setting `PYTHONPATH`{.interpreted-text role="envvar"} to an empty value was equivalent to setting it to `.`, which leads to confusion when reasoning by analogy to how `PATH`{.interpreted-text role="envvar"} works. The behavior now conforms to the posix convention for `PATH`{.interpreted-text role="envvar"}.
- The \[X refs, Y blocks\] output of a debug (`--with-pydebug`) build of the CPython interpreter is now off by default. It can be re-enabled using the `-X showrefcount` option. (Contributed by Ezio Melotti in `17323`{.interpreted-text role="issue"}.)
- The python command and most stdlib scripts (as well as `argparse`{.interpreted-text role="mod"}) now output `--version` information to `stdout` instead of `stderr` (for issue list see `other-improvements-3.4`{.interpreted-text role="ref"} above).

### Changes in the Python API

- The ABCs defined in `importlib.abc`{.interpreted-text role="mod"} now either raise the appropriate exception or return a default value instead of raising `NotImplementedError`{.interpreted-text role="exc"} blindly. This will only affect code calling `super`{.interpreted-text role="func"} and falling through all the way to the ABCs. For compatibility, catch both `NotImplementedError`{.interpreted-text role="exc"} or the appropriate exception as needed.
- The module type now initializes the `~module.__package__`{.interpreted-text role="attr"} and `~module.__loader__`{.interpreted-text role="attr"} attributes to `None` by default. To determine if these attributes were set in a backwards-compatible fashion, use e.g. `getattr(module, '__loader__', None) is not None`. (`17115`{.interpreted-text role="issue"}.)
- `!importlib.util.module_for_loader`{.interpreted-text role="meth"} now sets `__loader__` and `__package__` unconditionally to properly support reloading. If this is not desired then you will need to set these attributes manually. You can use `!importlib.util.module_to_load`{.interpreted-text role="func"} for module management.
- Import now resets relevant attributes (e.g. `__name__`, `__loader__`, `__package__`, `__file__`, `__cached__`) unconditionally when reloading. Note that this restores a pre-3.3 behavior in that it means a module is re-found when re-loaded (`19413`{.interpreted-text role="issue"}).
- Frozen packages no longer set `__path__` to a list containing the package name, they now set it to an empty list. The previous behavior could cause the import system to do the wrong thing on submodule imports if there was also a directory with the same name as the frozen package. The correct way to determine if a module is a package or not is to use `hasattr(module, '__path__')` (`18065`{.interpreted-text role="issue"}).
- Frozen modules no longer define a `__file__` attribute. It\'s semantically incorrect for frozen modules to set the attribute as they are not loaded from any explicit location. If you must know that a module comes from frozen code then you can see if the module\'s `__spec__.location` is set to `'frozen'`, check if the loader is a subclass of `importlib.machinery.FrozenImporter`{.interpreted-text role="class"}, or if Python 2 compatibility is necessary you can use `!imp.is_frozen`{.interpreted-text role="func"}.
- `py_compile.compile`{.interpreted-text role="func"} now raises `FileExistsError`{.interpreted-text role="exc"} if the file path it would write to is a symlink or a non-regular file. This is to act as a warning that import will overwrite those files with a regular file regardless of what type of file path they were originally.
- `importlib.abc.SourceLoader.get_source`{.interpreted-text role="meth"} no longer raises `ImportError`{.interpreted-text role="exc"} when the source code being loaded triggers a `SyntaxError`{.interpreted-text role="exc"} or `UnicodeDecodeError`{.interpreted-text role="exc"}. As `ImportError`{.interpreted-text role="exc"} is meant to be raised only when source code cannot be found but it should, it was felt to be over-reaching/overloading of that meaning when the source code is found but improperly structured. If you were catching ImportError before and wish to continue to ignore syntax or decoding issues, catch all three exceptions now.
- `functools.update_wrapper`{.interpreted-text role="func"} and `functools.wraps`{.interpreted-text role="func"} now correctly set the `__wrapped__` attribute to the function being wrapped, even if that function also had its `__wrapped__` attribute set. This means `__wrapped__` attributes now correctly link a stack of decorated functions rather than every `__wrapped__` attribute in the chain referring to the innermost function. Introspection libraries that assumed the previous behaviour was intentional can use `inspect.unwrap`{.interpreted-text role="func"} to access the first function in the chain that has no `__wrapped__` attribute.
- `inspect.getfullargspec`{.interpreted-text role="func"} has been reimplemented on top of `inspect.signature`{.interpreted-text role="func"} and hence handles a much wider variety of callable objects than it did in the past. It is expected that additional builtin and extension module callables will gain signature metadata over the course of the Python 3.4 series. Code that assumes that `inspect.getfullargspec`{.interpreted-text role="func"} will fail on non-Python callables may need to be adjusted accordingly.
- `importlib.machinery.PathFinder`{.interpreted-text role="class"} now passes on the current working directory to objects in `sys.path_hooks`{.interpreted-text role="data"} for the empty string. This results in `sys.path_importer_cache`{.interpreted-text role="data"} never containing `''`, thus iterating through `sys.path_importer_cache`{.interpreted-text role="data"} based on `sys.path`{.interpreted-text role="data"} will not find all keys. A module\'s `__file__` when imported in the current working directory will also now have an absolute path, including when using `-m` with the interpreter (except for `__main__.__file__` when a script has been executed directly using a relative path) (Contributed by Brett Cannon in `18416`{.interpreted-text role="issue"}). is specified on the command-line) (`18416`{.interpreted-text role="issue"}).
- The removal of the *strict* argument to `~http.client.HTTPConnection`{.interpreted-text role="class"} and `~http.client.HTTPSConnection`{.interpreted-text role="class"} changes the meaning of the remaining arguments if you are specifying them positionally rather than by keyword. If you\'ve been paying attention to deprecation warnings your code should already be specifying any additional arguments via keywords.
- Strings between `from __future__ import ...` statements now *always* raise a `SyntaxError`{.interpreted-text role="exc"}. Previously if there was no leading docstring, an interstitial string would sometimes be ignored. This brings CPython into compliance with the language spec; Jython and PyPy already were. (`17434`{.interpreted-text role="issue"}).
- `ssl.SSLSocket.getpeercert`{.interpreted-text role="meth"} and `ssl.SSLSocket.do_handshake`{.interpreted-text role="meth"} now raise an `OSError`{.interpreted-text role="exc"} with `ENOTCONN` when the `SSLSocket` is not connected, instead of the previous behavior of raising an `AttributeError`{.interpreted-text role="exc"}. In addition, `~ssl.SSLSocket.getpeercert`{.interpreted-text role="meth"} will raise a `ValueError`{.interpreted-text role="exc"} if the handshake has not yet been done.
- `base64.b32decode`{.interpreted-text role="func"} now raises a `binascii.Error`{.interpreted-text role="exc"} when the input string contains non-b32-alphabet characters, instead of a `TypeError`{.interpreted-text role="exc"}. This particular `TypeError`{.interpreted-text role="exc"} was missed when the other `TypeError`{.interpreted-text role="exc"}s were converted. (Contributed by Serhiy Storchaka in `18011`{.interpreted-text role="issue"}.) Note: this change was also inadvertently applied in Python 3.3.3.
- The `!file`{.interpreted-text role="attr"} attribute is now automatically closed when the creating `!cgi.FieldStorage`{.interpreted-text role="class"} instance is garbage collected. If you were pulling the file object out separately from the `!cgi.FieldStorage`{.interpreted-text role="class"} instance and not keeping the instance alive, then you should either store the entire `!cgi.FieldStorage`{.interpreted-text role="class"} instance or read the contents of the file before the `!cgi.FieldStorage`{.interpreted-text role="class"} instance is garbage collected.
- Calling `read` or `write` on a closed SSL socket now raises an informative `ValueError`{.interpreted-text role="exc"} rather than the previous more mysterious `AttributeError`{.interpreted-text role="exc"} (`9177`{.interpreted-text role="issue"}).
- `slice.indices`{.interpreted-text role="meth"} no longer produces an `OverflowError`{.interpreted-text role="exc"} for huge values. As a consequence of this fix, `slice.indices`{.interpreted-text role="meth"} now raises a `ValueError`{.interpreted-text role="exc"} if given a negative length; previously it returned nonsense values (`14794`{.interpreted-text role="issue"}).
- The `complex`{.interpreted-text role="class"} constructor, unlike the `cmath`{.interpreted-text role="mod"} functions, was incorrectly accepting `float`{.interpreted-text role="class"} values if an object\'s `__complex__` special method returned one. This now raises a `TypeError`{.interpreted-text role="exc"}. (`16290`{.interpreted-text role="issue"}.)
- The `int`{.interpreted-text role="class"} constructor in 3.2 and 3.3 erroneously accepts `float`{.interpreted-text role="class"} values for the *base* parameter. It is unlikely anyone was doing this, but if so, it will now raise a `TypeError`{.interpreted-text role="exc"} (`16772`{.interpreted-text role="issue"}).
- Defaults for keyword-only arguments are now evaluated *after* defaults for regular keyword arguments, instead of before. Hopefully no one wrote any code that depends on the previous buggy behavior (`16967`{.interpreted-text role="issue"}).
- Stale thread states are now cleared after `~os.fork`{.interpreted-text role="func"}. This may cause some system resources to be released that previously were incorrectly kept perpetually alive (for example, database connections kept in thread-local storage). (`17094`{.interpreted-text role="issue"}.)
- Parameter names in `__annotations__` dicts are now mangled properly, similarly to `~function.__kwdefaults__`{.interpreted-text role="attr"}. (Contributed by Yury Selivanov in `20625`{.interpreted-text role="issue"}.)
- `hashlib.hash.name`{.interpreted-text role="attr"} now always returns the identifier in lower case. Previously some builtin hashes had uppercase names, but now that it is a formal public interface the naming has been made consistent (`18532`{.interpreted-text role="issue"}).
- Because `unittest.TestSuite`{.interpreted-text role="mod"} now drops references to tests after they are run, test harnesses that reuse a `~unittest.TestSuite`{.interpreted-text role="class"} to re-run a set of tests may fail. Test suites should not be re-used in this fashion since it means state is retained between test runs, breaking the test isolation that `unittest`{.interpreted-text role="mod"} is designed to provide. However, if the lack of isolation is considered acceptable, the old behavior can be restored by creating a `~unittest.TestSuite`{.interpreted-text role="mod"} subclass that defines a `_removeTestAtIndex` method that does nothing (see `.TestSuite.__iter__`{.interpreted-text role="meth"}) (`11798`{.interpreted-text role="issue"}).
- `unittest`{.interpreted-text role="mod"} now uses `argparse`{.interpreted-text role="mod"} for command line parsing. There are certain invalid command forms that used to work that are no longer allowed; in theory this should not cause backward compatibility issues since the disallowed command forms didn\'t make any sense and are unlikely to be in use.
- The `re.split`{.interpreted-text role="func"}, `re.findall`{.interpreted-text role="func"}, and `re.sub`{.interpreted-text role="func"} functions, and the `~re.Match.group`{.interpreted-text role="meth"} and `~re.Match.groups`{.interpreted-text role="meth"} methods of `match` objects now always return a *bytes* object when the string to be matched is a `bytes-like object`{.interpreted-text role="term"}. Previously the return type matched the input type, so if your code was depending on the return value being, say, a `bytearray`, you will need to change your code.
- `!audioop`{.interpreted-text role="mod"} functions now raise an error immediately if passed string input, instead of failing randomly later on (`16685`{.interpreted-text role="issue"}).
- The new *convert_charrefs* argument to `~html.parser.HTMLParser`{.interpreted-text role="class"} currently defaults to `False` for backward compatibility, but will eventually be changed to default to `True`. It is recommended that you add this keyword, with the appropriate value, to any `~html.parser.HTMLParser`{.interpreted-text role="class"} calls in your code (`13633`{.interpreted-text role="issue"}).
- Since the *digestmod* argument to the `hmac.new`{.interpreted-text role="func"} function will in the future have no default, all calls to `hmac.new`{.interpreted-text role="func"} should be changed to explicitly specify a *digestmod* (`17276`{.interpreted-text role="issue"}).
- Calling `sysconfig.get_config_var`{.interpreted-text role="func"} with the `SO` key, or looking `SO` up in the results of a call to `sysconfig.get_config_vars`{.interpreted-text role="func"} is deprecated. This key should be replaced by `EXT_SUFFIX` or `SHLIB_SUFFIX`, depending on the context (`19555`{.interpreted-text role="issue"}).
- Any calls to `open` functions that specify `U` should be modified. `U` is ineffective in Python3 and will eventually raise an error if used. Depending on the function, the equivalent of its old Python2 behavior can be achieved using either a *newline* argument, or if necessary by wrapping the stream in `~io.TextIOWrapper`{.interpreted-text role="mod"} to use its *newline* argument (`15204`{.interpreted-text role="issue"}).
- If you use `pyvenv` in a script and desire that pip *not* be installed, you must add `--without-pip` to your command invocation.
- The default behavior of `json.dump`{.interpreted-text role="func"} and `json.dumps`{.interpreted-text role="func"} when an indent is specified has changed: it no longer produces trailing spaces after the item separating commas at the ends of lines. This will matter only if you have tests that are doing white-space-sensitive comparisons of such output (`16333`{.interpreted-text role="issue"}).
- `doctest`{.interpreted-text role="mod"} now looks for doctests in extension module `__doc__` strings, so if your doctest test discovery includes extension modules that have things that look like doctests in them you may see test failures you\'ve never seen before when running your tests (`3158`{.interpreted-text role="issue"}).
- The `collections.abc`{.interpreted-text role="mod"} module has been slightly refactored as part of the Python startup improvements. As a consequence of this, it is no longer the case that importing `collections`{.interpreted-text role="mod"} automatically imports `collections.abc`{.interpreted-text role="mod"}. If your program depended on the (undocumented) implicit import, you will need to add an explicit `import collections.abc` (`20784`{.interpreted-text role="issue"}).

### Changes in the C API

- `PyEval_EvalFrameEx`{.interpreted-text role="c:func"}, `PyObject_Repr`{.interpreted-text role="c:func"}, and `PyObject_Str`{.interpreted-text role="c:func"}, along with some other internal C APIs, now include a debugging assertion that ensures they are not used in situations where they may silently discard a currently active exception. In cases where discarding the active exception is expected and desired (for example, because it has already been saved locally with `PyErr_Fetch`{.interpreted-text role="c:func"} or is being deliberately replaced with a different exception), an explicit `PyErr_Clear`{.interpreted-text role="c:func"} call will be needed to avoid triggering the assertion when invoking these operations (directly or indirectly) and running against a version of Python that is compiled with assertions enabled.
- `PyErr_SetImportError`{.interpreted-text role="c:func"} now sets `TypeError`{.interpreted-text role="exc"} when its **msg** argument is not set. Previously only `NULL` was returned with no exception set.
- The result of the `PyOS_ReadlineFunctionPointer`{.interpreted-text role="c:data"} callback must now be a string allocated by `PyMem_RawMalloc`{.interpreted-text role="c:func"} or `PyMem_RawRealloc`{.interpreted-text role="c:func"}, or `NULL` if an error occurred, instead of a string allocated by `PyMem_Malloc`{.interpreted-text role="c:func"} or `PyMem_Realloc`{.interpreted-text role="c:func"} (`16742`{.interpreted-text role="issue"})
- `PyThread_set_key_value`{.interpreted-text role="c:func"} now always set the value. In Python 3.3, the function did nothing if the key already exists (if the current value is a non-`NULL` pointer).
- The `f_tstate` (thread state) field of the `PyFrameObject`{.interpreted-text role="c:type"} structure has been removed to fix a bug: see `14432`{.interpreted-text role="issue"} for the rationale.

## Changed in 3.4.3

### PEP 476: Enabling certificate verification by default for stdlib http clients {#pep-476}

`http.client`{.interpreted-text role="mod"} and modules which use it, such as `urllib.request`{.interpreted-text role="mod"} and `xmlrpc.client`{.interpreted-text role="mod"}, will now verify that the server presents a certificate which is signed by a CA in the platform trust store and whose hostname matches the hostname being requested by default, significantly improving security for many applications.

For applications which require the old previous behavior, they can pass an alternate context:

    import urllib.request
    import ssl

    # This disables all verification
    context = ssl._create_unverified_context()

    # This allows using a specific certificate for the host, which doesn't need
    # to be in the trust store
    context = ssl.create_default_context(cafile="/path/to/file.crt")

    urllib.request.urlopen("https://invalid-cert", context=context)