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What's New In Python 3.0

Author : Guido van Rossum

This article explains the new features in Python 3.0, compared to 2.6. Python 3.0, also known as "Python 3000" or "Py3K", is the first ever intentionally backwards incompatible Python release. Python 3.0 was released on December 3, 2008. There are more changes than in a typical release, and more that are important for all Python users. Nevertheless, after digesting the changes, you'll find that Python really hasn't changed all that much -- by and large, we're mostly fixing well-known annoyances and warts, and removing a lot of old cruft.

This article doesn't attempt to provide a complete specification of all new features, but instead tries to give a convenient overview. For full details, you should refer to the documentation for Python 3.0, and/or the many PEPs referenced in the text. If you want to understand the complete implementation and design rationale for a particular feature, PEPs usually have more details than the regular documentation; but note that PEPs usually are not kept up-to-date once a feature has been fully implemented.

Due to time constraints this document is not as complete as it should have been. As always for a new release, the Misc/NEWS file in the source distribution contains a wealth of detailed information about every small thing that was changed.

Common Stumbling Blocks

This section lists those few changes that are most likely to trip you up if you're used to Python 2.5.

Print Is A Function

The print statement has been replaced with a print{.interpreted-text role="func"} function, with keyword arguments to replace most of the special syntax of the old print statement (3105{.interpreted-text role="pep"}). Examples:

Old: print "The answer is", 2*2
New: print("The answer is", 2*2)

Old: print x,           # Trailing comma suppresses newline
New: print(x, end=" ")  # Appends a space instead of a newline

Old: print              # Prints a newline
New: print()            # You must call the function!

Old: print >>sys.stderr, "fatal error"
New: print("fatal error", file=sys.stderr)

Old: print (x, y)       # prints repr((x, y))
New: print((x, y))      # Not the same as print(x, y)!

You can also customize the separator between items, e.g.:

print("There are <", 2**32, "> possibilities!", sep="")

which produces:

There are <4294967296> possibilities!

Note:

The print{.interpreted-text role="func"} function doesn't support the "softspace" feature of the old print statement. For example, in Python 2.x, print "A\n", "B" would write "A\nB\n"; but in Python 3.0, print("A\n", "B") writes "A\n B\n".
Initially, you'll be finding yourself typing the old print x a lot in interactive mode. Time to retrain your fingers to type print(x) instead!
When using the 2to3 source-to-source conversion tool, all print statements are automatically converted to print{.interpreted-text role="func"} function calls, so this is mostly a non-issue for larger projects.

Views And Iterators Instead Of Lists

Some well-known APIs no longer return lists:

dict{.interpreted-text role="class"} methods dict.keys{.interpreted-text role="meth"}, dict.items{.interpreted-text role="meth"} and dict.values{.interpreted-text role="meth"} return "views" instead of lists. For example, this no longer works: k = d.keys(); k.sort(). Use k = sorted(d) instead (this works in Python 2.5 too and is just as efficient).
Also, the !dict.iterkeys{.interpreted-text role="meth"}, !dict.iteritems{.interpreted-text role="meth"} and !dict.itervalues{.interpreted-text role="meth"} methods are no longer supported.
map{.interpreted-text role="func"} and filter{.interpreted-text role="func"} return iterators. If you really need a list and the input sequences are all of equal length, a quick fix is to wrap map{.interpreted-text role="func"} in list{.interpreted-text role="func"}, e.g. list(map(...)), but a better fix is often to use a list comprehension (especially when the original code uses lambda{.interpreted-text role="keyword"}), or rewriting the code so it doesn't need a list at all. Particularly tricky is map{.interpreted-text role="func"} invoked for the side effects of the function; the correct transformation is to use a regular for{.interpreted-text role="keyword"} loop (since creating a list would just be wasteful).

If the input sequences are not of equal length, map{.interpreted-text role="func"} will stop at the termination of the shortest of the sequences. For full compatibility with map{.interpreted-text role="func"} from Python 2.x, also wrap the sequences in itertools.zip_longest{.interpreted-text role="func"}, e.g. map(func, *sequences) becomes list(map(func, itertools.zip_longest(*sequences))).
range{.interpreted-text role="func"} now behaves like !xrange{.interpreted-text role="func"} used to behave, except it works with values of arbitrary size. The latter no longer exists.
zip{.interpreted-text role="func"} now returns an iterator.

Ordering Comparisons

Python 3.0 has simplified the rules for ordering comparisons:

The ordering comparison operators (<, <=, >=, >) raise a TypeError exception when the operands don't have a meaningful natural ordering. Thus, expressions like 1 < '', 0 > None or len <= len are no longer valid, and e.g. None < None raises TypeError{.interpreted-text role="exc"} instead of returning False. A corollary is that sorting a heterogeneous list no longer makes sense -- all the elements must be comparable to each other. Note that this does not apply to the == and != operators: objects of different incomparable types always compare unequal to each other.
sorted{.interpreted-text role="meth"} and list.sort{.interpreted-text role="meth"} no longer accept the cmp argument providing a comparison function. Use the key argument instead. N.B. the key and reverse arguments are now "keyword-only".
The !cmp{.interpreted-text role="func"} function should be treated as gone, and the !__cmp__{.interpreted-text role="meth"} special method is no longer supported. Use ~object.__lt__{.interpreted-text role="meth"} for sorting, ~object.__eq__{.interpreted-text role="meth"} with ~object.__hash__{.interpreted-text role="meth"}, and other rich comparisons as needed. (If you really need the !cmp{.interpreted-text role="func"} functionality, you could use the expression (a > b) - (a < b) as the equivalent for cmp(a, b).)

Integers

237{.interpreted-text role="pep"}: Essentially, !long{.interpreted-text role="class"} renamed to int{.interpreted-text role="class"}. That is, there is only one built-in integral type, named int{.interpreted-text role="class"}; but it behaves mostly like the old !long{.interpreted-text role="class"} type.
238{.interpreted-text role="pep"}: An expression like 1/2 returns a float. Use 1//2 to get the truncating behavior. (The latter syntax has existed for years, at least since Python 2.2.)
The !sys.maxint{.interpreted-text role="data"} constant was removed, since there is no longer a limit to the value of integers. However, sys.maxsize{.interpreted-text role="data"} can be used as an integer larger than any practical list or string index. It conforms to the implementation's "natural" integer size and is typically the same as !sys.maxint{.interpreted-text role="data"} in previous releases on the same platform (assuming the same build options).
The repr{.interpreted-text role="func"} of a long integer doesn't include the trailing L anymore, so code that unconditionally strips that character will chop off the last digit instead. (Use str{.interpreted-text role="func"} instead.)
Octal literals are no longer of the form 0720; use 0o720 instead.

Text Vs. Data Instead Of Unicode Vs. 8-bit

Everything you thought you knew about binary data and Unicode has changed.

Python 3.0 uses the concepts of text and (binary) data instead of Unicode strings and 8-bit strings. All text is Unicode; however encoded Unicode is represented as binary data. The type used to hold text is str{.interpreted-text role="class"}, the type used to hold data is bytes{.interpreted-text role="class"}. The biggest difference with the 2.x situation is that any attempt to mix text and data in Python 3.0 raises TypeError{.interpreted-text role="exc"}, whereas if you were to mix Unicode and 8-bit strings in Python 2.x, it would work if the 8-bit string happened to contain only 7-bit (ASCII) bytes, but you would get UnicodeDecodeError{.interpreted-text role="exc"} if it contained non-ASCII values. This value-specific behavior has caused numerous sad faces over the years.
As a consequence of this change in philosophy, pretty much all code that uses Unicode, encodings or binary data most likely has to change. The change is for the better, as in the 2.x world there were numerous bugs having to do with mixing encoded and unencoded text. To be prepared in Python 2.x, start using !unicode{.interpreted-text role="class"} for all unencoded text, and str{.interpreted-text role="class"} for binary or encoded data only. Then the 2to3 tool will do most of the work for you.
You can no longer use u"..." literals for Unicode text. However, you must use b"..." literals for binary data.
As the str{.interpreted-text role="class"} and bytes{.interpreted-text role="class"} types cannot be mixed, you must always explicitly convert between them. Use str.encode{.interpreted-text role="meth"} to go from str{.interpreted-text role="class"} to bytes{.interpreted-text role="class"}, and bytes.decode{.interpreted-text role="meth"} to go from bytes{.interpreted-text role="class"} to str{.interpreted-text role="class"}. You can also use bytes(s, encoding=...) and str(b, encoding=...), respectively.
Like str{.interpreted-text role="class"}, the bytes{.interpreted-text role="class"} type is immutable. There is a separate mutable type to hold buffered binary data, bytearray{.interpreted-text role="class"}. Nearly all APIs that accept bytes{.interpreted-text role="class"} also accept bytearray{.interpreted-text role="class"}. The mutable API is based on collections.MutableSequence <collections.abc.MutableSequence>{.interpreted-text role="class"}.
All backslashes in raw string literals are interpreted literally. This means that '\U' and '\u' escapes in raw strings are not treated specially. For example, r'\u20ac' is a string of 6 characters in Python 3.0, whereas in 2.6, ur'\u20ac' was the single "euro" character. (Of course, this change only affects raw string literals; the euro character is '\u20ac' in Python 3.0.)
The built-in !basestring{.interpreted-text role="class"} abstract type was removed. Use str{.interpreted-text role="class"} instead. The str{.interpreted-text role="class"} and bytes{.interpreted-text role="class"} types don't have functionality enough in common to warrant a shared base class. The 2to3 tool (see below) replaces every occurrence of !basestring{.interpreted-text role="class"} with str{.interpreted-text role="class"}.
Files opened as text files (still the default mode for open{.interpreted-text role="func"}) always use an encoding to map between strings (in memory) and bytes (on disk). Binary files (opened with a b in the mode argument) always use bytes in memory. This means that if a file is opened using an incorrect mode or encoding, I/O will likely fail loudly, instead of silently producing incorrect data. It also means that even Unix users will have to specify the correct mode (text or binary) when opening a file. There is a platform-dependent default encoding, which on Unixy platforms can be set with the LANG environment variable (and sometimes also with some other platform-specific locale-related environment variables). In many cases, but not all, the system default is UTF-8; you should never count on this default. Any application reading or writing more than pure ASCII text should probably have a way to override the encoding. There is no longer any need for using the encoding-aware streams in the codecs{.interpreted-text role="mod"} module.
The initial values of sys.stdin{.interpreted-text role="data"}, sys.stdout{.interpreted-text role="data"} and sys.stderr{.interpreted-text role="data"} are now unicode-only text files (i.e., they are instances of io.TextIOBase{.interpreted-text role="class"}). To read and write bytes data with these streams, you need to use their io.TextIOBase.buffer{.interpreted-text role="data"} attribute.
Filenames are passed to and returned from APIs as (Unicode) strings. This can present platform-specific problems because on some platforms filenames are arbitrary byte strings. (On the other hand, on Windows filenames are natively stored as Unicode.) As a work-around, most APIs (e.g. open{.interpreted-text role="func"} and many functions in the os{.interpreted-text role="mod"} module) that take filenames accept bytes{.interpreted-text role="class"} objects as well as strings, and a few APIs have a way to ask for a bytes{.interpreted-text role="class"} return value. Thus, os.listdir{.interpreted-text role="func"} returns a list of bytes{.interpreted-text role="class"} instances if the argument is a bytes{.interpreted-text role="class"} instance, and os.getcwdb{.interpreted-text role="func"} returns the current working directory as a bytes{.interpreted-text role="class"} instance. Note that when os.listdir{.interpreted-text role="func"} returns a list of strings, filenames that cannot be decoded properly are omitted rather than raising UnicodeError{.interpreted-text role="exc"}.
Some system APIs like os.environ{.interpreted-text role="data"} and sys.argv{.interpreted-text role="data"} can also present problems when the bytes made available by the system is not interpretable using the default encoding. Setting the LANG variable and rerunning the program is probably the best approach.
3138{.interpreted-text role="pep"}: The repr{.interpreted-text role="func"} of a string no longer escapes non-ASCII characters. It still escapes control characters and code points with non-printable status in the Unicode standard, however.
3120{.interpreted-text role="pep"}: The default source encoding is now UTF-8.
3131{.interpreted-text role="pep"}: Non-ASCII letters are now allowed in identifiers. (However, the standard library remains ASCII-only with the exception of contributor names in comments.)
The !StringIO{.interpreted-text role="mod"} and !cStringIO{.interpreted-text role="mod"} modules are gone. Instead, import the io{.interpreted-text role="mod"} module and use io.StringIO{.interpreted-text role="class"} or io.BytesIO{.interpreted-text role="class"} for text and data respectively.
See also the unicode-howto{.interpreted-text role="ref"}, which was updated for Python 3.0.

Overview Of Syntax Changes

This section gives a brief overview of every syntactic change in Python 3.0.

New Syntax

3107{.interpreted-text role="pep"}: Function argument and return value annotations. This provides a standardized way of annotating a function's parameters and return value. There are no semantics attached to such annotations except that they can be introspected at runtime using the ~object.__annotations__{.interpreted-text role="attr"} attribute. The intent is to encourage experimentation through metaclasses, decorators or frameworks.
3102{.interpreted-text role="pep"}: Keyword-only arguments. Named parameters occurring after *args in the parameter list must be specified using keyword syntax in the call. You can also use a bare * in the parameter list to indicate that you don't accept a variable-length argument list, but you do have keyword-only arguments.
Keyword arguments are allowed after the list of base classes in a class definition. This is used by the new convention for specifying a metaclass (see next section), but can be used for other purposes as well, as long as the metaclass supports it.
3104{.interpreted-text role="pep"}: nonlocal{.interpreted-text role="keyword"} statement. Using nonlocal x you can now assign directly to a variable in an outer (but non-global) scope. !nonlocal{.interpreted-text role="keyword"} is a new reserved word.
3132{.interpreted-text role="pep"}: Extended Iterable Unpacking. You can now write things like a, b, *rest = some_sequence. And even *rest, a = stuff. The rest object is always a (possibly empty) list; the right-hand side may be any iterable. Example:
```
(a, *rest, b) = range(5)
```
This sets a to 0, b to 4, and rest to [1, 2, 3].
Dictionary comprehensions: {k: v for k, v in stuff} means the same thing as dict(stuff) but is more flexible. (This is 274{.interpreted-text role="pep"} vindicated. :-)
Set literals, e.g. {1, 2}. Note that {} is an empty dictionary; use set() for an empty set. Set comprehensions are also supported; e.g., {x for x in stuff} means the same thing as set(stuff) but is more flexible.
New octal literals, e.g. 0o720 (already in 2.6). The old octal literals (0720) are gone.
New binary literals, e.g. 0b1010 (already in 2.6), and there is a new corresponding built-in function, bin{.interpreted-text role="func"}.
Bytes literals are introduced with a leading b or B, and there is a new corresponding built-in function, bytes{.interpreted-text role="func"}.

Changed Syntax

3109{.interpreted-text role="pep"} and 3134{.interpreted-text role="pep"}: new raise{.interpreted-text role="keyword"} statement syntax: raise [{expr} [from {expr}]]{.interpreted-text role="samp"}. See below.
!as{.interpreted-text role="keyword"} and with{.interpreted-text role="keyword"} are now reserved words. (Since 2.6, actually.)
True, False, and None are reserved words. (2.6 partially enforced the restrictions on None already.)
Change from except{.interpreted-text role="keyword"} exc, var to !except{.interpreted-text role="keyword"} exc !as{.interpreted-text role="keyword"} var. See 3110{.interpreted-text role="pep"}.
3115{.interpreted-text role="pep"}: New Metaclass Syntax. Instead of:
```
class C:
    __metaclass__ = M
    ...
```
you must now use:
```
class C(metaclass=M):
    ...
```
The module-global !__metaclass__{.interpreted-text role="data"} variable is no longer supported. (It was a crutch to make it easier to default to new-style classes without deriving every class from object{.interpreted-text role="class"}.)
List comprehensions no longer support the syntactic form [... for {var} in {item1}, {item2}, ...]{.interpreted-text role="samp"}. Use [... for {var} in ({item1}, {item2}, ...)]{.interpreted-text role="samp"} instead. Also note that list comprehensions have different semantics: they are closer to syntactic sugar for a generator expression inside a list{.interpreted-text role="func"} constructor, and in particular the loop control variables are no longer leaked into the surrounding scope.
The ellipsis (...) can be used as an atomic expression anywhere. (Previously it was only allowed in slices.) Also, it must now be spelled as .... (Previously it could also be spelled as . . ., by a mere accident of the grammar.)

Removed Syntax

3113{.interpreted-text role="pep"}: Tuple parameter unpacking removed. You can no longer write def foo(a, (b, c)): .... Use def foo(a, b_c): b, c = b_c instead.
Removed backticks (use repr{.interpreted-text role="func"} instead).
Removed <> (use != instead).
Removed keyword: exec{.interpreted-text role="func"} is no longer a keyword; it remains as a function. (Fortunately the function syntax was also accepted in 2.x.) Also note that exec{.interpreted-text role="func"} no longer takes a stream argument; instead of exec(f) you can use exec(f.read()).
Integer literals no longer support a trailing l or L.
String literals no longer support a leading u or U.
The from{.interpreted-text role="keyword"} module import{.interpreted-text role="keyword"} * syntax is only allowed at the module level, no longer inside functions.
The only acceptable syntax for relative imports is from .[{module}] import {name}{.interpreted-text role="samp"}. All import{.interpreted-text role="keyword"} forms not starting with . are interpreted as absolute imports. (328{.interpreted-text role="pep"})
Classic classes are gone.

Changes Already Present In Python 2.6

Since many users presumably make the jump straight from Python 2.5 to Python 3.0, this section reminds the reader of new features that were originally designed for Python 3.0 but that were back-ported to Python 2.6. The corresponding sections in whats-new-in-2.6{.interpreted-text role="ref"} should be consulted for longer descriptions.

pep-0343{.interpreted-text role="ref"}. The with{.interpreted-text role="keyword"} statement is now a standard feature and no longer needs to be imported from the __future__{.interpreted-text role="mod"}. Also check out new-26-context-managers{.interpreted-text role="ref"} and new-module-contextlib{.interpreted-text role="ref"}.
pep-0366{.interpreted-text role="ref"}. This enhances the usefulness of the -m{.interpreted-text role="option"} option when the referenced module lives in a package.
pep-0370{.interpreted-text role="ref"}.
pep-0371{.interpreted-text role="ref"}.
pep-3101{.interpreted-text role="ref"}. Note: the 2.6 description mentions the format{.interpreted-text role="meth"} method for both 8-bit and Unicode strings. In 3.0, only the str{.interpreted-text role="class"} type (text strings with Unicode support) supports this method; the bytes{.interpreted-text role="class"} type does not. The plan is to eventually make this the only API for string formatting, and to start deprecating the % operator in Python 3.1.
pep-3105{.interpreted-text role="ref"}. This is now a standard feature and no longer needs to be imported from __future__{.interpreted-text role="mod"}. More details were given above.
pep-3110{.interpreted-text role="ref"}. The except{.interpreted-text role="keyword"} exc !as{.interpreted-text role="keyword"} var syntax is now standard and !except{.interpreted-text role="keyword"} exc, var is no longer supported. (Of course, the !as{.interpreted-text role="keyword"} var part is still optional.)
pep-3112{.interpreted-text role="ref"}. The b"..." string literal notation (and its variants like b'...', b"""...""", and br"...") now produces a literal of type bytes{.interpreted-text role="class"}.
pep-3116{.interpreted-text role="ref"}. The io{.interpreted-text role="mod"} module is now the standard way of doing file I/O. The built-in open{.interpreted-text role="func"} function is now an alias for io.open{.interpreted-text role="func"} and has additional keyword arguments encoding, errors, newline and closefd. Also note that an invalid mode argument now raises ValueError{.interpreted-text role="exc"}, not IOError{.interpreted-text role="exc"}. The binary file object underlying a text file object can be accessed as !f.buffer{.interpreted-text role="attr"} (but beware that the text object maintains a buffer of itself in order to speed up the encoding and decoding operations).
pep-3118{.interpreted-text role="ref"}. The old builtin !buffer{.interpreted-text role="func"} is now really gone; the new builtin memoryview{.interpreted-text role="func"} provides (mostly) similar functionality.
pep-3119{.interpreted-text role="ref"}. The abc{.interpreted-text role="mod"} module and the ABCs defined in the collections{.interpreted-text role="mod"} module plays a somewhat more prominent role in the language now, and built-in collection types like dict{.interpreted-text role="class"} and list{.interpreted-text role="class"} conform to the collections.MutableMapping <collections.abc.MutableMapping>{.interpreted-text role="class"} and collections.MutableSequence <collections.abc.MutableSequence>{.interpreted-text role="class"} ABCs, respectively.
pep-3127{.interpreted-text role="ref"}. As mentioned above, the new octal literal notation is the only one supported, and binary literals have been added.
pep-3129{.interpreted-text role="ref"}.
pep-3141{.interpreted-text role="ref"}. The numbers{.interpreted-text role="mod"} module is another new use of ABCs, defining Python's "numeric tower". Also note the new fractions{.interpreted-text role="mod"} module which implements numbers.Rational{.interpreted-text role="class"}.

Library Changes

Due to time constraints, this document does not exhaustively cover the very extensive changes to the standard library. 3108{.interpreted-text role="pep"} is the reference for the major changes to the library. Here's a capsule review:

Many old modules were removed. Some, like !gopherlib{.interpreted-text role="mod"} (no longer used) and !md5{.interpreted-text role="mod"} (replaced by hashlib{.interpreted-text role="mod"}), were already deprecated by 4{.interpreted-text role="pep"}. Others were removed as a result of the removal of support for various platforms such as Irix, BeOS and Mac OS 9 (see 11{.interpreted-text role="pep"}). Some modules were also selected for removal in Python 3.0 due to lack of use or because a better replacement exists. See 3108{.interpreted-text role="pep"} for an exhaustive list.
The !bsddb3{.interpreted-text role="mod"} package was removed because its presence in the core standard library has proved over time to be a particular burden for the core developers due to testing instability and Berkeley DB's release schedule. However, the package is alive and well, externally maintained at https://www.jcea.es/programacion/pybsddb.htm.
Some modules were renamed because their old name disobeyed 8{.interpreted-text role="pep"}, or for various other reasons. Here's the list:

Old Name New Name

[winreg]{#winreg} winreg ConfigParser configparser copy_reg copyreg Queue queue SocketServer socketserver markupbase [markupbase]{#markupbase} repr reprlib test.test_support test.support
A common pattern in Python 2.x is to have one version of a module implemented in pure Python, with an optional accelerated version implemented as a C extension; for example, pickle{.interpreted-text role="mod"} and !cPickle{.interpreted-text role="mod"}. This places the burden of importing the accelerated version and falling back on the pure Python version on each user of these modules. In Python 3.0, the accelerated versions are considered implementation details of the pure Python versions. Users should always import the standard version, which attempts to import the accelerated version and falls back to the pure Python version. The pickle{.interpreted-text role="mod"} / !cPickle{.interpreted-text role="mod"} pair received this treatment. The profile{.interpreted-text role="mod"} module is on the list for 3.1. The !StringIO{.interpreted-text role="mod"} module has been turned into a class in the io{.interpreted-text role="mod"} module.
Some related modules have been grouped into packages, and usually the submodule names have been simplified. The resulting new packages are:
- dbm{.interpreted-text role="mod"} (!anydbm{.interpreted-text role="mod"}, !dbhash{.interpreted-text role="mod"}, !dbm{.interpreted-text role="mod"}, !dumbdbm{.interpreted-text role="mod"}, !gdbm{.interpreted-text role="mod"}, !whichdb{.interpreted-text role="mod"}).
- html{.interpreted-text role="mod"} (!HTMLParser{.interpreted-text role="mod"}, !htmlentitydefs{.interpreted-text role="mod"}).
- http{.interpreted-text role="mod"} (!httplib{.interpreted-text role="mod"}, !BaseHTTPServer{.interpreted-text role="mod"}, !CGIHTTPServer{.interpreted-text role="mod"}, !SimpleHTTPServer{.interpreted-text role="mod"}, !Cookie{.interpreted-text role="mod"}, !cookielib{.interpreted-text role="mod"}).
- tkinter{.interpreted-text role="mod"} (all Tkinter-related modules except turtle{.interpreted-text role="mod"}). The target audience of turtle{.interpreted-text role="mod"} doesn't really care about tkinter{.interpreted-text role="mod"}. Also note that as of Python 2.6, the functionality of turtle{.interpreted-text role="mod"} has been greatly enhanced.
- urllib{.interpreted-text role="mod"} (!urllib{.interpreted-text role="mod"}, !urllib2{.interpreted-text role="mod"}, !urlparse{.interpreted-text role="mod"}, !robotparse{.interpreted-text role="mod"}).
- xmlrpc{.interpreted-text role="mod"} (!xmlrpclib{.interpreted-text role="mod"}, !DocXMLRPCServer{.interpreted-text role="mod"}, !SimpleXMLRPCServer{.interpreted-text role="mod"}).

Some other changes to standard library modules, not covered by 3108{.interpreted-text role="pep"}:

Killed !sets{.interpreted-text role="mod"}. Use the built-in set{.interpreted-text role="func"} class.
Cleanup of the sys{.interpreted-text role="mod"} module: removed !sys.exitfunc{.interpreted-text role="func"}, !sys.exc_clear{.interpreted-text role="func"}, !sys.exc_type{.interpreted-text role="data"}, !sys.exc_value{.interpreted-text role="data"}, !sys.exc_traceback{.interpreted-text role="data"}. (Note that sys.last_type{.interpreted-text role="data"} etc. remain.)
Cleanup of the array.array{.interpreted-text role="class"} type: the !read{.interpreted-text role="meth"} and !write{.interpreted-text role="meth"} methods are gone; use ~array.array.fromfile{.interpreted-text role="meth"} and ~array.array.tofile{.interpreted-text role="meth"} instead. Also, the 'c' typecode for array is gone -- use either 'b' for bytes or 'u' for Unicode characters.
Cleanup of the operator{.interpreted-text role="mod"} module: removed !sequenceIncludes{.interpreted-text role="func"} and !isCallable{.interpreted-text role="func"}.
Cleanup of the !thread{.interpreted-text role="mod"} module: !acquire_lock{.interpreted-text role="func"} and !release_lock{.interpreted-text role="func"} are gone; use ~threading.Lock.acquire{.interpreted-text role="meth"} and ~threading.Lock.release{.interpreted-text role="meth"} instead.
Cleanup of the random{.interpreted-text role="mod"} module: removed the !jumpahead{.interpreted-text role="func"} API.
The !new{.interpreted-text role="mod"} module is gone.
The functions !os.tmpnam{.interpreted-text role="func"}, !os.tempnam{.interpreted-text role="func"} and !os.tmpfile{.interpreted-text role="func"} have been removed in favor of the tempfile{.interpreted-text role="mod"} module.
The tokenize{.interpreted-text role="mod"} module has been changed to work with bytes. The main entry point is now tokenize.tokenize{.interpreted-text role="func"}, instead of generate_tokens.
!string.letters{.interpreted-text role="data"} and its friends (!string.lowercase{.interpreted-text role="data"} and !string.uppercase{.interpreted-text role="data"}) are gone. Use string.ascii_letters{.interpreted-text role="data"} etc. instead. (The reason for the removal is that !string.letters{.interpreted-text role="data"} and friends had locale-specific behavior, which is a bad idea for such attractively named global "constants".)
Renamed module !__builtin__{.interpreted-text role="mod"} to builtins{.interpreted-text role="mod"} (removing the underscores, adding an 's'). The !__builtins__{.interpreted-text role="data"} variable found in most global namespaces is unchanged. To modify a builtin, you should use builtins{.interpreted-text role="mod"}, not !__builtins__{.interpreted-text role="data"}!

`3101`{.interpreted-text role="pep"}: A New Approach To String Formatting

A new system for built-in string formatting operations replaces the % string formatting operator. (However, the % operator is still supported; it will be deprecated in Python 3.1 and removed from the language at some later time.) Read 3101{.interpreted-text role="pep"} for the full scoop.

Changes To Exceptions

The APIs for raising and catching exception have been cleaned up and new powerful features added:

352{.interpreted-text role="pep"}: All exceptions must be derived (directly or indirectly) from BaseException{.interpreted-text role="exc"}. This is the root of the exception hierarchy. This is not new as a recommendation, but the requirement to inherit from BaseException{.interpreted-text role="exc"} is new. (Python 2.6 still allowed classic classes to be raised, and placed no restriction on what you can catch.) As a consequence, string exceptions are finally truly and utterly dead.
Almost all exceptions should actually derive from Exception{.interpreted-text role="exc"}; BaseException{.interpreted-text role="exc"} should only be used as a base class for exceptions that should only be handled at the top level, such as SystemExit{.interpreted-text role="exc"} or KeyboardInterrupt{.interpreted-text role="exc"}. The recommended idiom for handling all exceptions except for this latter category is to use except{.interpreted-text role="keyword"} Exception{.interpreted-text role="exc"}.
!StandardError{.interpreted-text role="exc"} was removed.
Exceptions no longer behave as sequences. Use the ~BaseException.args{.interpreted-text role="attr"} attribute instead.
3109{.interpreted-text role="pep"}: Raising exceptions. You must now use raise {Exception}({args}){.interpreted-text role="samp"} instead of raise {Exception}, {args}{.interpreted-text role="samp"}. Additionally, you can no longer explicitly specify a traceback; instead, if you have to do this, you can assign directly to the ~BaseException.__traceback__{.interpreted-text role="attr"} attribute (see below).
3110{.interpreted-text role="pep"}: Catching exceptions. You must now use except {SomeException} as {variable}{.interpreted-text role="samp"} instead of except {SomeException}, {variable}{.interpreted-text role="samp"}. Moreover, the variable is explicitly deleted when the except{.interpreted-text role="keyword"} block is left.
3134{.interpreted-text role="pep"}: Exception chaining. There are two cases: implicit chaining and explicit chaining. Implicit chaining happens when an exception is raised in an except{.interpreted-text role="keyword"} or finally{.interpreted-text role="keyword"} handler block. This usually happens due to a bug in the handler block; we call this a secondary exception. In this case, the original exception (that was being handled) is saved as the ~BaseException.__context__{.interpreted-text role="attr"} attribute of the secondary exception. Explicit chaining is invoked with this syntax:
```
raise SecondaryException() from primary_exception
```
(where primary_exception is any expression that produces an exception object, probably an exception that was previously caught). In this case, the primary exception is stored on the ~BaseException.__cause__{.interpreted-text role="attr"} attribute of the secondary exception. The traceback printed when an unhandled exception occurs walks the chain of !__cause__{.interpreted-text role="attr"} and ~BaseException.__context__{.interpreted-text role="attr"} attributes and prints a separate traceback for each component of the chain, with the primary exception at the top. (Java users may recognize this behavior.)
3134{.interpreted-text role="pep"}: Exception objects now store their traceback as the ~BaseException.__traceback__{.interpreted-text role="attr"} attribute. This means that an exception object now contains all the information pertaining to an exception, and there are fewer reasons to use sys.exc_info{.interpreted-text role="func"} (though the latter is not removed).
A few exception messages are improved when Windows fails to load an extension module. For example, error code 193 is now %1 is not a valid Win32 application. Strings now deal with non-English locales.

Miscellaneous Other Changes

Operators And Special Methods

!= now returns the opposite of ==, unless == returns NotImplemented{.interpreted-text role="data"}.
The concept of "unbound methods" has been removed from the language. When referencing a method as a class attribute, you now get a plain function object.
!__getslice__{.interpreted-text role="meth"}, !__setslice__{.interpreted-text role="meth"} and !__delslice__{.interpreted-text role="meth"} were killed. The syntax a[i:j] now translates to a.__getitem__(slice(i, j)) (or ~object.__setitem__{.interpreted-text role="meth"} or ~object.__delitem__{.interpreted-text role="meth"}, when used as an assignment or deletion target, respectively).
3114{.interpreted-text role="pep"}: the standard next{.interpreted-text role="meth"} method has been renamed to ~iterator.__next__{.interpreted-text role="meth"}.
The !__oct__{.interpreted-text role="meth"} and !__hex__{.interpreted-text role="meth"} special methods are removed -- oct{.interpreted-text role="func"} and hex{.interpreted-text role="func"} use ~object.__index__{.interpreted-text role="meth"} now to convert the argument to an integer.
Removed support for !__members__{.interpreted-text role="attr"} and !__methods__{.interpreted-text role="attr"}.
The function attributes named !func_X{.interpreted-text role="attr"} have been renamed to use the !__X__{.interpreted-text role="attr"} form, freeing up these names in the function attribute namespace for user-defined attributes. To wit, !func_closure{.interpreted-text role="attr"}, !func_code{.interpreted-text role="attr"}, !func_defaults{.interpreted-text role="attr"}, !func_dict{.interpreted-text role="attr"}, !func_doc{.interpreted-text role="attr"}, !func_globals{.interpreted-text role="attr"}, !func_name{.interpreted-text role="attr"} were renamed to ~function.__closure__{.interpreted-text role="attr"}, ~function.__code__{.interpreted-text role="attr"}, ~function.__defaults__{.interpreted-text role="attr"}, ~function.__dict__{.interpreted-text role="attr"}, ~function.__doc__{.interpreted-text role="attr"}, ~function.__globals__{.interpreted-text role="attr"}, ~function.__name__{.interpreted-text role="attr"}, respectively.
!__nonzero__{.interpreted-text role="meth"} is now ~object.__bool__{.interpreted-text role="meth"}.

Builtins

3135{.interpreted-text role="pep"}: New super{.interpreted-text role="func"}. You can now invoke super{.interpreted-text role="func"} without arguments and (assuming this is in a regular instance method defined inside a class{.interpreted-text role="keyword"} statement) the right class and instance will automatically be chosen. With arguments, the behavior of super{.interpreted-text role="func"} is unchanged.
3111{.interpreted-text role="pep"}: !raw_input{.interpreted-text role="func"} was renamed to input{.interpreted-text role="func"}. That is, the new input{.interpreted-text role="func"} function reads a line from sys.stdin{.interpreted-text role="data"} and returns it with the trailing newline stripped. It raises EOFError{.interpreted-text role="exc"} if the input is terminated prematurely. To get the old behavior of input{.interpreted-text role="func"}, use eval(input()).
A new built-in function next{.interpreted-text role="func"} was added to call the ~iterator.__next__{.interpreted-text role="meth"} method on an object.
The round{.interpreted-text role="func"} function rounding strategy and return type have changed. Exact halfway cases are now rounded to the nearest even result instead of away from zero. (For example, round(2.5) now returns 2 rather than 3.) round(x[, n]) now delegates to x.__round__([n]) instead of always returning a float. It generally returns an integer when called with a single argument and a value of the same type as x when called with two arguments.
Moved !intern{.interpreted-text role="func"} to sys.intern{.interpreted-text role="func"}.
Removed: !apply{.interpreted-text role="func"}. Instead of apply(f, args) use f(*args).
Removed callable{.interpreted-text role="func"}. Instead of callable(f) you can use isinstance(f, collections.Callable). The !operator.isCallable{.interpreted-text role="func"} function is also gone.
Removed !coerce{.interpreted-text role="func"}. This function no longer serves a purpose now that classic classes are gone.
Removed !execfile{.interpreted-text role="func"}. Instead of execfile(fn) use exec(open(fn).read()).
Removed the !file{.interpreted-text role="class"} type. Use open{.interpreted-text role="func"}. There are now several different kinds of streams that open can return in the io{.interpreted-text role="mod"} module.
Removed !reduce{.interpreted-text role="func"}. Use functools.reduce{.interpreted-text role="func"} if you really need it; however, 99 percent of the time an explicit for{.interpreted-text role="keyword"} loop is more readable.
Removed !reload{.interpreted-text role="func"}. Use !imp.reload{.interpreted-text role="func"}.
Removed. !dict.has_key{.interpreted-text role="meth"} -- use the in{.interpreted-text role="keyword"} operator instead.

Build and C API Changes

Due to time constraints, here is a very incomplete list of changes to the C API.

Support for several platforms was dropped, including but not limited to Mac OS 9, BeOS, RISCOS, Irix, and Tru64.
3118{.interpreted-text role="pep"}: New Buffer API.
3121{.interpreted-text role="pep"}: Extension Module Initialization & Finalization.
3123{.interpreted-text role="pep"}: Making PyObject_HEAD{.interpreted-text role="c:macro"} conform to standard C.
No more C API support for restricted execution.
!PyNumber_Coerce{.interpreted-text role="c:func"}, !PyNumber_CoerceEx{.interpreted-text role="c:func"}, !PyMember_Get{.interpreted-text role="c:func"}, and !PyMember_Set{.interpreted-text role="c:func"} C APIs are removed.
New C API !PyImport_ImportModuleNoBlock{.interpreted-text role="c:func"}, works like PyImport_ImportModule{.interpreted-text role="c:func"} but won't block on the import lock (returning an error instead).
Renamed the boolean conversion C-level slot and method: nb_nonzero is now nb_bool.
Removed !METH_OLDARGS{.interpreted-text role="c:macro"} and !WITH_CYCLE_GC{.interpreted-text role="c:macro"} from the C API.

Performance

The net result of the 3.0 generalizations is that Python 3.0 runs the pystone benchmark around 10% slower than Python 2.5. Most likely the biggest cause is the removal of special-casing for small integers. There's room for improvement, but it will happen after 3.0 is released!

Porting To Python 3.0

For porting existing Python 2.5 or 2.6 source code to Python 3.0, the best strategy is the following:

(Prerequisite:) Start with excellent test coverage.
Port to Python 2.6. This should be no more work than the average port from Python 2.x to Python 2.(x+1). Make sure all your tests pass.
(Still using 2.6:) Turn on the !-3{.interpreted-text role="option"} command line switch. This enables warnings about features that will be removed (or change) in 3.0. Run your test suite again, and fix code that you get warnings about until there are no warnings left, and all your tests still pass.
Run the 2to3 source-to-source translator over your source code tree. Run the result of the translation under Python 3.0. Manually fix up any remaining issues, fixing problems until all tests pass again.

It is not recommended to try to write source code that runs unchanged under both Python 2.6 and 3.0; you'd have to use a very contorted coding style, e.g. avoiding print statements, metaclasses, and much more. If you are maintaining a library that needs to support both Python 2.6 and Python 3.0, the best approach is to modify step 3 above by editing the 2.6 version of the source code and running the 2to3 translator again, rather than editing the 3.0 version of the source code.

For porting C extensions to Python 3.0, please see cporting-howto{.interpreted-text role="ref"}.