cpython_15/instruction.md

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---

# PEP 793: PyModExport: A new entry point for C extension modules

## Abstract
In this PEP, we propose a new entry point for C extension modules, by which
one can define a module using an array of `PyModuleDef_Slot` structures
without an enclosing `PyModuleDef` structure.
This allows extension authors to avoid using a statically allocated
`PyObject`, lifting the most common obstacle to making one compiled library
file usable with both regular and free-threaded builds of CPython.

To make this viable, we also specify new module slot types to replace
`PyModuleDef`'s fields, and to allow adding a *token* similar to the
`Py_tp_token` used for type objects.

We also add an API for defining modules from slots dynamically.

The existing API (`PyInit_*`) is soft-deprecated.
(That is: it will continue to work without warnings, and it'll be fully
documented and supported, but we plan to not add any new features to it.)

## Background & Motivation
The memory layout of Python objects differs between regular and free-threading
builds.
So, an ABI that supports both regular and free-threading builds cannot include
the current `PyObject` memory layout. To stay compatible with existing ABI
(and API), it cannot support statically allocated Python objects.

There is one type of object that is needed in most extension modules
and is allocated statically in virtually all cases: the `PyModuleDef` returned
from the module export hooks (that is, `PyInit_*` functions).

Module export hooks (`PyInit_*` functions) can return two kinds of objects:

1. A fully initialized module object (for so-called
   *single-phase initialization*). This was the only option in 3.4 and below.
   Modules created this way have surprising (but backwards-compatible)
   behaviour around multiple interpreters or repeated loading.
   (Specifically, the *contents* of such a module's `__dict__` are shared
   across all instances of the module object.)

   The module returned is typically created using the `PyModule_Create`
   function, which requires a statically allocated (or at least long-lived)
   `PyModuleDef` struct.

   It is possible to bypass this using the lower-level `PyModule_New*` API.
   This avoids the need for `PyModuleDef`, but offers much less functionality.

2. A `PyModuleDef` object containing a description of how to create a module
   object. This option, *multi-phase initialization*, was introduced in
   PEP 489; see its motivation for why it exists.

The interpreter cannot distinguish between these cases before the export hook
is called.

### The interpreter switch
Python 3.12 added a way for modules to mark whether they may be
loaded in a subinterpreter: the `Py_mod_multiple_interpreters` slot.
Setting it to the “not supported” value signals that an extension
can only be loaded in the main interpreter.

Unfortunately, Python can only get this information by *calling* the
module export hook.
For single-phase modules, that creates the module object and runs arbitrary
initialization code.
For modules that set `Py_mod_multiple_interpreters` to “not supported”,
this initialization needs to happen in the main interpreter.

To make this work, if a new module is loaded in a sub-interpreter, Python
temporarily switches to the main interpreter, calls the export hook
there, and then either switches back and redoes the import, or fails.

This unnecessary and fragile extra work highlights the underlying design issue:
Python has no way to get information about an extension
before the extension can, potentially, fully initialize itself.

## Rationale
For avoiding the module export hook requiring a statically allocated
`PyObject*`, two options come to mind:

- Returning a *dynamically* allocated object, whose ownership is transferred
  to the interpreter. This stucture could be very similar to the existing
  `PyModuleDef`, since it needs to contain the same data.
  Unlike the existing `PyModuleDef`, this one would need to be
  reference-counted so that it both outlives “its” module and does not leak.

- Adding a new export hook, which does not return a `PyObject*`.

  This was considered already for Python 3.5 in PEP 489, but rejected:

    Keeping only the PyInit hook name, even if it’s not entirely appropriate
    for exporting a definition, yielded a much simpler solution.

  Alas, after a decade of fixing the implications of this choice, the solution
  is no longer simple.

A new hook will also allow Python to avoid the second issue mentioned in
Motivation -- the interpreter switch.
Effectivelly, it will add a new phase to multi-phase initialization, in which
Python can check whether the module is compatible.

### Using slots without a wrapper struct
The existing `PyModuleDef` is a struct with some fixed fields and
a “slots” array.
Unlike slots, the fixed fields cannot be individually deprecated and replaced.
This proposal does away with fixed fields and proposes using a slots array
directly, without a wrapper struct.

The `PyModuleDef_Slot` struct does have some downsides compared to fixed fields.
We believe these are fixable, but leave that out of scope of this PEP
(see “Improving slots in general” in the Possible Future Directions section).

### Tokens
A static `PyModuleDef` has another purpose besides describing
how a module should be created.
As a statically allocated singleton that remains attached to the module object,
it allows extension authors to check whether a given Python module is “theirs”:
if a module object has a known `PyModuleDef`, its module state will have
a known memory layout.

An analogous issue was solved for types by adding `Py_tp_token`.
This proposal adds the same mechanism to modules.

Unlike types, the import mechanism often has a pointer that's known to be
suitable as a token value; in these cases it can provide a default token.
Thus, module tokens do not need a variant of the inelegant `Py_TP_USE_SPEC`.

To help extensions that straddle Python versions, `PyModuleDef` addresses
are used as default tokens, and where it's reasonable, they are made
interchangeable with tokens.

### Soft-deprecating the existing export hook
The only reason for authors of *existing* extensions to switch to the
API proposed here is that it allows a single module for both free-threaded
and non-free-threaded builds.
It is important that Python *allows* that, but for many existing modules,
it is nowhere near worth losing compatibility with 3.14 and lower versions.

It is much too early to plan deprecation of the old API.

Instead, this PEP proposes to stop adding new features to the `PyInit_*`
scheme.
After all, the perfect time for extension authors to switch is when they want
to modify module initialization anyway.

## Specification
### The export hook
When importing an extension module, Python will now first look for an export hook
like this:

```c
PyModuleDef_Slot *PyModExport_<NAME>(void);
```
where `<NAME>` is the name of the module.
For non-ASCII names, it will instead look for `PyModExportU_<NAME>`,
with `<NAME>` encoded as for existing `PyInitU_*` hooks
(that is, *punycode*-encoded with hyphens replaced by underscores).

If not found, the import will continue as in previous Python versions (that is,
by looking up a `PyInit_*` or `PyInitU_*` function).

If found, Python will call the hook with no arguments.

On failure, the export hook must return NULL with an exception set.
This will cause the import to fail.
(Python will not fall back to `PyInit_*` on error.)

On success, the hook must return a pointer to an array of
`PyModuleDef_Slot` structs.
Python will then create a module based on the given slots by calling functions
proposed below: `PyModule_FromSlotsAndSpec` and `PyModule_Exec`.
See their description for requirements on the slots array.

The returned array and all data it points to (recursively) must remain valid
and constant until runtime shutdown.
(We expect functions to export a static constant, or one of several constants
chosen depending on, for example, `Py_Version`. Dynamic behaviour should
generally happen in the `Py_mod_create` and `Py_mod_exec` functions.)

### Dynamic creation
A new function will be added to create a module from an array of slots:

```c
PyObject *PyModule_FromSlotsAndSpec(const PyModuleDef_Slot *slots, PyObject *spec)
```
The *slots* argument must point to an array of `PyModuleDef_Slot` structures,
terminated by a slot with `slot=0` (typically written as `{0}` in C).
There are no required slots, though *slots* must not be `NULL`.
It follows that minimal input contains only the terminator slot.

> **Note:** If PEP 803 is accepted, the `Py_mod_abi` slot will be mandatory.
The *spec* argument is a duck-typed ModuleSpec-like object, meaning that any
attributes defined for `importlib.machinery.ModuleSpec` have matching
semantics.
The `name` attribute is required, but this limitation may be lifted in the
future.
The `name` will be used *instead of* the `Py_mod_name` slot (just like
`PyModule_FromDefAndSpec` ignores `PyModuleDef.m_name`).

The slots arrays for both `PyModule_FromSlotsAndSpec` and the new export hook
will only allow up to one `Py_mod_exec` slot.
Arrays in `PyModuleDef.m_slots` may have more; this will not change.
This limitation is easy to work around and multiple `exec` slots are rarely
used [#multiexec]_.

For modules created without a `PyModuleDef`, the `Py_mod_create` function
will be called with `NULL` for the second argument (*def*).
(In the future, if we find a use case for passing the input slots array, a new
slot with an updated signature can be added.)

Unlike the `PyModExport_*` hook, the *slots* array may be changed or
destroyed after the `PyModule_FromSlotsAndSpec` call.
(That is, Python must take a copy of all input data.)
As an exception, any `PyMethodDef` array given by `Py_mod_methods`
must be statically allocated (or be otherwise guaranteed to outlive the
objects created from it). This limitation may be lifted in the future.

A new function, `PyModule_Exec`, will be added to run the `exec` slot(s) for a module.
This acts like `PyModule_ExecDef`, but supports modules created using slots,
and does not take an explicit *def*:

```c
int PyModule_Exec(PyObject *module)
```
Calling this is required to fully initialize a module.
`PyModule_FromSlotsAndSpec` will *not* run it (just like
`PyModule_FromDefAndSpec` does not call `PyModule_ExecDef`).

For modules created from a *def*, calling this is equivalent to
calling `PyModule_ExecDef(module, PyModule_GetDef(module))`.

### Tokens
Module objects will optionally store a “token”: a `void*` pointer
similar to `Py_tp_token` for types.

> **Note:** This is specialized functionality meant replace the `PyType_GetModuleByDef` function; users that don't need `PyType_GetModuleByDef` will most likely not need tokens either. This section contains the technical specification; for an example of intended usage, see `exampletype_repr` in the `Example section <pep793-example>`.
If specified, using a new `Py_mod_token` slot, the module token must:

- outlive the module, so it's not reused for something else while the module
  exists; and
- "belong" to the extension module where the module lives, so it will not
  clash with other extension modules.

(Typically, it should be the slots array or `PyModuleDef` that a module is
created from, or another static constant for dynamically created modules.)

When the address of a `PyModuleDef` is used as a module's token,
the module should behave as if it was created from that `PyModuleDef`.
In particular, the module state must have matching layout and semantics.

Modules created using the `PyModule_FromSlotsAndSpec` or the
`PyModExport_<NAME>` export hook can use a new `Py_mod_token` slot
to set the token.

Modules created from a `PyModuleDef` will have the token set to that
definition. An explicit `Py_mod_token` slot will we rejected for these.
(This allows implementations to share storage for the token and def.)

For modules created via the new export hook, the token
will be set to the address of the slots array by default.
(This does **not** apply to modules created by `PyModule_FromSlotsAndSpec`,
as that function's input might not outlive the module.)

The token will not be set for non-`PyModuleType` instances.

A `PyModule_GetToken` function will be added to get the token.
Since the result may be `NULL`, it will be passed via a pointer; the function
will return 0 on success and -1 on failure:

```c
int PyModule_GetToken(PyObject *, void **token_p)
```
A new `PyType_GetModuleByToken` function will be added, with a signature
like the existing `PyType_GetModuleByDef` but a `const void *token` argument,
and the same behaviour except matching tokens rather than only defs,
and returning a strong reference.

For easier backwards compatibility, the existing `PyType_GetModuleByDef`
will be changed to also allow a token (cast to a `PyModuleDef *` pointer) as
the *def* argument.
That is, `PyType_GetModuleByToken` and `PyType_GetModuleByDef` will differ
only in the formal signature of the second argument and by returning a
borrowed vs. strong reference.
(The `PyModule_GetDef` function will not get a similar change, as users may
access members of its result.)

### New slots
For each field of the `PyModuleDef` struct, except ones from
`PyModuleDef_HEAD_INIT`, a new slot ID will be provided: `Py_mod_name`,
`Py_mod_doc`, `Py_mod_clear`, etc.
Slots related to the module state rather than the module object will
use a `Py_mod_state_` prefix.
See `pep793-api-summary` for a full list.

All new slots -- these and `Py_tp_token` discussed above -- may not be
repeated in the slots array, and may not be used in a
`PyModuleDef.m_slots` array.
They may not have a `NULL` value (instead, the slot can be omitted entirely).

Note that currently, for modules created from a *spec* (that is, using
`PyModule_FromDefAndSpec`), the `PyModuleDef.m_name` member is ignored
and the name from the spec is used instead.
All API proposed in this document creates modules from a *spec*, and it will
ignore `Py_mod_name` in the same way.
The slot will be optional, but extension authors are strongly encouraged to
include it for the benefit of future APIs, external tooling, debugging,
and introspection.

### Bits & Pieces
A `PyMODEXPORT_FUNC` macro will be added, similar to the `PyMODINIT_FUNC`
macro but with `PyModuleDef_Slot *` as the return type.

A `PyModule_GetStateSize` function will be added to retrieve the size set
by `Py_mod_state_size` or `PyModuleDef.m_size`.
Since the result may be -1 (for single-phase-init modules), it will be output
via a pointer; the function will return 0 on success and -1 on failure:

```c
int PyModule_GetStateSize(PyObject *, Py_ssize_t *result);
```
### Soft-deprecating the existing export hook
The `PyInit_*` export hook will be
`soft-deprecated <pep387-soft-deprecation>`.

### New API summary
Python will load a new module export hook, with two variants:

```c
PyModuleDef_Slot *PyModExport_<NAME>(void);
PyModuleDef_Slot *PyModExportU_<ENCODED_NAME>(void);
```
The following functions will be added:

```c
PyObject *PyModule_FromSlotsAndSpec(const PyModuleDef_Slot *, PyObject *spec)
int PyModule_Exec(PyObject *)
int PyModule_GetToken(PyObject *, void**)
PyObject *PyType_GetModuleByToken(PyTypeObject *type, const void *token)
int PyModule_GetStateSize(PyObject *, Py_ssize_t *result);
```
A new macro will be added:

```c
PyMODEXPORT_FUNC
```
And new slot types (`#define`\d names for small integers):

- `Py_mod_name` (equivalent to `PyModuleDef.m_name`)
- `Py_mod_doc` (equivalent to `PyModuleDef.m_doc`)
- `Py_mod_state_size` (equivalent to `PyModuleDef.m_size`)
- `Py_mod_methods` (equivalent to `PyModuleDef.m_methods`)
- `Py_mod_state_traverse` (equivalent to `PyModuleDef.m_traverse`)
- `Py_mod_state_clear` (equivalent to `PyModuleDef.m_clear`)
- `Py_mod_state_free` (equivalent to `PyModuleDef.m_free`)
- `Py_mod_token` (see above)

All this will be added to the Limited API.

## Backwards Compatibility
If an existing module is ported to use the new mechanism, then
`PyModule_GetDef` will start returning `NULL` for it.
(This matches `PyModule_GetDef`'s current documentation.)
We claim that how a module was defined is an implementation detail of that
module, so this should not be considered a breaking change.

Similarly, the `PyType_GetModuleByDef` function may stop matching modules
whose definition changed. Module authors may avoid this by explicitly
setting a *def* as the *token*.

`PyType_GetModuleByDef` will now accept a module token as the *def* argument.
We specify a suitable restriction on using `PyModuleDef` addresses as tokens,
and non-`PyModuleDef` pointers were previously invalid input,
so this is not a backwards-compatibility issue.

The `Py_mod_create` function may now be called with `NULL` for the second
argument.
This could trip people porting from *def* to *slots*, so it needs to be
mentioned in porting notes.

### Forward compatibility
If a module defines the new export hook, CPython versions that implement this
PEP will ignore the traditional `PyInit_*` hook.

Extensions that straddle Python versions are expected to define both hooks;
each build of CPython will “pick” the newest one that it supports.

### Porting guide
Here is a guide to convert an existing module to the new API, including
some tricky edge cases.
It should be moved to a HOWTO in the documentation.

This guide is meant for hand-written modules. For code generators and language
wrappers, the `pep793-shim` below may be more useful.

#. Scan your code for uses of `PyModule_GetDef`. This function will
   return `NULL` for modules that use the new mechanism. Instead:

   * For getting the contents of the module's `PyModuleDef`, use the C struct
     directly. Alternatively, get attributes from the module using, for
     example, `PyModule_GetNameObject`, the `__doc__` attribute, and
     `PyModule_GetStateSize`.
     (Note that Python code can mutate a module's attributes.)
   * For testing if a module object is “yours”, use `PyModule_GetToken`
     instead.
     Later in this guide, you'll set the token to *be* the existing
     `PyModuleDef` structure.

#. Optionally, scan your code for uses of `PyType_GetModuleByDef`,
   and replace them with `PyType_GetModuleByToken`.
   Later in this guide, you'll set the token to *be* the existing
   `PyModuleDef` structure.

   (You may skip this step if targetting Python versions that don't expose
   `PyType_GetModuleByToken`, since `PyType_GetModuleByDef` is
   backwards-compatible.)

#. Look at the function identified by `Py_mod_create`, if any.
   Make sure that it does not use its second argument (`PyModuleDef`),
   as it will be called with `NULL`.
   Instead of the argument, use the existing `PyModuleDef` struct directly.
#. If using multiple `Py_mod_exec` slots, consolidate them: pick one of
   the functions, or write a new one, and call the others from it.
   Remove all but one `Py_mod_exec` slots.
#. Make a copy of the existing `PyModuleDef_Slot` array pointed to by
   the `m_slots` member of your `PyModuleDef`. If you don't have an
   existing slots array, create one like this:

   ```c
   static PyModuleDef_Slot module_slots[] = {
       {0}
   };
   ```

   Give this array a unique name.
   Further examples will assume that you've named it `module_slots`.

#. Add slots for all members of the existing `PyModuleDef` structure.
   See `pep793-api-summary` for a list of the new slots.
   For example, to add a name and docstring:

   ```c
   static PyModuleDef_Slot module_slots[] = {
       {Py_mod_name, "mymodule"},
       {Py_mod_doc, (char*)PyDoc_STR("my docstring")},
       // ... (keep existing slots here)
       {0}
   };
   ```

#. If you switched from `PyModule_GetDef` to `PyModule_GetToken`,
   and/or if you use `PyType_GetModuleByDef` or `PyType_GetModuleByToken`,
   add a `Py_mod_token` slot pointing to the existing `PyModuleDef` struct:

   ```c
   static PyModuleDef_Slot module_slots[] = {
       // ... (keep existing slots here)
       {Py_mod_token, &your_module_def},
       {0}
   };
   ```

#. Add a new export hook.

   ```c
   PyMODEXPORT_FUNC PyModExport_examplemodule(PyObject);

   PyMODEXPORT_FUNC
   PyModExport_examplemodule(void)
   {
       return module_slots;
   }
   ```

The new export hook will be used on Python 3.15 and above.
Once your module no longer supports lower versions:

#. Delete the `PyInit_` function.

#. If the existing `PyModuleDef` struct is used *only* for `Py_mod_token`
   and/or `PyType_GetModuleByToken`, you may remove the `Py_mod_token`
   line and replace `&your_module_def` with `module_slots` everywhere else.

#. Delete any unused data.
   The `PyModuleDef` struct and the original slots array are likely to be
   unused.

### Backwards compatibility shim
It is possible to write a generic function that implements the “old” export
hook (`PyInit_`) in terms of the API proposed here.

The following implementation can be copied and pasted to a project; only the
names `PyInit_examplemodule` (twice) and `PyModExport_examplemodule` should
need adjusting.

When added to the `pep793-example` below and compiled with a
non-free-threaded build of this PEP's reference implementation, the resulting
extension is compatible with non-free-threading 3.9+ builds, in addition to a
free-threading build of the reference implementation.
(The module must be named without a version tag, e.g. `examplemodule.so`,
and be placed on `sys.path`.)

Full support for creating such modules will require backports of some new
API, and support in build/install tools. This is out of scope of this PEP.
(In particular, the demo “cheats” by using a subset of Limited API 3.15 that
*happens to work* on 3.9; a proper implementation would use Limited API 3.9
with backport shims for new API like `Py_mod_name`.)

This implementation places a few additional requirements on the slots array:

- Slots that correspond to `PyModuleDef` members must come first.
- A `Py_mod_name` slot is required.
- Any `Py_mod_token` must be set to `&module_def_and_token`, defined here.

_(see `pep-0793/shim.c` for the full listing)_:language: c

## Security Implications
None known

## How to Teach This
In addition to regular reference docs, the `pep793-porting-notes` should
be added as a new HOWTO.

## Example
_(see `pep-0793/examplemodule.c` for the full listing)_:language: c

## Rejected Ideas
### Exporting a data pointer rather than a function
This proposes a new module export *function*, which is expected to
return static constant data.
That data could be exported directly as a data pointer.

With a function, we avoid dealing with a new kind of exported symbol.

A function also allows the extension to introspect its environment in a limited
way -- for example, to tailor the returned data to the current Python version.

### Changing `PyModuleDef` to not be `PyObject`
It is possible to change `PyModuleDef` to no longer include the `PyObject`
header, and continue using the current `PyInit_*` hook.
There are several issues with this approach:

- The import machinery would need to examine bit-patterns in the objects to
  distinguish between different memory layouts:

  - the “old” `PyObject`-based `PyModuleDef`, returned by current `abi3`
    extensions,
  - the new `PyModuleDef`,
  - `PyObject`-based module objects, for single-phase initialization.

  This is fragile, and places constraints on future changes to `PyObject`:
  the memory layouts need to stay *distinguishable* until both single-phase
  initialization and the current Stable ABI are no longer supported.

- `PyModuleDef_Init` is documented to “Ensure a module definition is a
  properly initialized Python object that correctly reports its type and
  a reference count.”
  This would need to change without warning, breaking any user code that treats
  `PyModuleDef`\ s as Python objects.

## Possible Future Directions
These ideas are out of scope for *this* proposal.

### Improving slots in general
Slots -- and specifically the existing `PyModuleDef_Slot` -- do have a few
shortcomings. The most important are:

- Type safety: `void *` is used for data pointers, function pointers
  and small integers, requiring casting that is technically undefined
  behaviour in C -- but works in practice on all relevant architectures.
  (For example: `Py_tp_doc` marks a string; `Py_mod_gil` an integer.)

- Limited forward compatibility: if an extension provides a slot ID that's
  unknown to the current interpreter, module creation will fail.
  This makes it cumbersome to use “optional” features -- ones that should only
  take effect if the interpreter supports them.
  (The recently added slots `Py_mod_gil` and `Py_mod_multiple_interpreters`
  are good examples.)

  One workaround is to check `Py_Version` in the export function,
  and return a slot array suitable for the current interpreter.

### Updating defaults
With a new API, we could update defaults for the
`Py_mod_multiple_interpreters` and `Py_mod_gil` slots.

### The inittab
We'll need to allow `PyModuleDef`-less slots in the inittab --
that is, add a new variant of `PyImport_ExtendInittab`.
Should that be part of this PEP?

The inittab is used for embedding, where a common/stable ABI is not that
important. So, it might be OK to leave this to a later change.

## Linked PEP 489 — Multi-phase extension module initialization
### Abstract

This PEP proposes a redesign of the way in which built-in and extension modules interact with the import machinery. This was last revised for Python 3.0 in PEP 3121, but did not solve all problems at the time. The goal is to solve import-related problems by bringing extension modules closer to the way Python modules behave; specifically to hook into the ModuleSpec-based loading mechanism introduced in PEP 451.

This proposal draws inspiration from `PyType_Spec` of PEP 384 to allow extension authors to only define features they need, and to allow future additions to extension module declarations.

Extensions modules are created in a two-step process, fitting better into the ModuleSpec architecture, with parallels to `__new__` and `__init__` of classes.

Extension modules can safely store arbitrary C-level per-module state in the module that is covered by normal garbage collection and supports reloading and sub-interpreters. Extension authors are encouraged to take these issues into account when using the new API.

The proposal also allows extension modules with non-ASCII names.

Not all problems tackled in PEP 3121 are solved in this proposal. In particular, problems with run-time module lookup (`PyState_FindModule`) are left to a future PEP.

### Motivation

Python modules and extension modules are not being set up in the same way. For Python modules, the module object is created and set up first, then the module code is being executed (PEP 302). A ModuleSpec object (PEP 451) is used to hold information about the module, and passed to the relevant hooks.

For extensions (i.e. shared libraries) and built-in modules, the module init function is executed straight away and does both the creation and initialization. The initialization function is not passed the ModuleSpec, or any information it contains, such as the `__file__` or fully-qualified name. This hinders relative imports and resource loading.

In Py3, modules are also not being added to sys.modules, which means that a (potentially transitive) re-import of the module will really try to re-import it and thus run into an infinite loop when it executes the module init function again. Without access to the fully-qualified module name, it is not trivial to correctly add the module to sys.modules either. This is specifically a problem for Cython generated modules, for which it's not uncommon that the module init code has the same level of complexity as that of any 'regular' Python module. Also, the lack of `__file__` and `__name__` information hinders the compilation of "`__init__.py`" modules, i.e. packages, especially when relative imports are being used at module init time.

Furthermore, the majority of currently existing extension modules has problems with sub-interpreter support and/or interpreter reloading, and, while it is possible with the current infrastructure to support these features, it is neither easy nor efficient. Addressing these issues was the goal of PEP 3121, but many extensions, including some in the standard library, took the least-effort approach to porting to Python 3, leaving these issues unresolved. This PEP keeps backwards compatibility, which should reduce pressure and give extension authors adequate time to consider these issues when porting.

### The current process

Currently, extension and built-in modules export an initialization function named "`PyInit_modulename`", named after the file name of the shared library. This function is executed by the import machinery and must return a fully initialized module object. The function receives no arguments, so it has no way of knowing about its import context.

During its execution, the module init function creates a module object based on a `PyModuleDef` object. It then continues to initialize it by adding attributes to the module dict, creating types, etc.

In the back, the shared library loader keeps a note of the fully qualified module name of the last module that it loaded, and when a module gets created that has a matching name, this global variable is used to determine the fully qualified name of the module object. This is not entirely safe as it relies on the module init function creating its own module object first, but this assumption usually holds in practice.

### The proposal

The initialization function (`PyInit_modulename`) will be allowed to return a pointer to a `PyModuleDef` object. The import machinery will be in charge of constructing the module object, calling hooks provided in the `PyModuleDef` in the relevant phases of initialization (as described below).

This multi-phase initialization is an additional possibility. Single-phase initialization, the current practice of returning a fully initialized module object, will still be accepted, so existing code will work unchanged, including binary compatibility.

The `PyModuleDef` structure will be changed to contain a list of slots, similarly to PEP 384's `PyType_Spec` for types. To keep binary compatibility, and avoid needing to introduce a new structure (which would introduce additional supporting functions and per-module storage), the currently unused *m_reload* pointer of `PyModuleDef` will be changed to hold the slots. The structures are defined as:

```c
typedef struct {
    int slot;
    void *value;
} PyModuleDef_Slot;

typedef struct PyModuleDef {
    PyModuleDef_Base m_base;
    const char* m_name;
    const char* m_doc;
    Py_ssize_t m_size;
    PyMethodDef *m_methods;
    PyModuleDef_Slot *m_slots;  /* changed from `inquiry m_reload;` */
    traverseproc m_traverse;
    inquiry m_clear;
    freefunc m_free;
} PyModuleDef;
```

The *m_slots* member must be either NULL, or point to an array of `PyModuleDef_Slot` structures, terminated by a slot with id set to 0 (i.e. `{0, NULL}`).

To specify a slot, a unique slot ID must be provided. New Python versions may introduce new slot IDs, but slot IDs will never be recycled. Slots may get deprecated, but will continue to be supported throughout Python 3.x.

A slot's value pointer may not be NULL, unless specified otherwise in the slot's documentation.

The following slots are currently available, and described later:

* `Py_mod_create`
* `Py_mod_exec`

Unknown slot IDs will cause the import to fail with SystemError.

When using multi-phase initialization, the *m_name* field of `PyModuleDef` will not be used during importing; the module name will be taken from the ModuleSpec.

Before it is returned from `PyInit_*`, the `PyModuleDef` object must be initialized using the newly added `PyModuleDef_Init` function. This sets the object type (which cannot be done statically on certain compilers), refcount, and internal bookkeeping data (*m_index*). For example, an extension module "example" would be exported as:

```c
static PyModuleDef example_def = {...}

PyMODINIT_FUNC
PyInit_example(void)
{
    return PyModuleDef_Init(&example_def);
}
```

The `PyModuleDef` object must be available for the lifetime of the module created from it -- usually, it will be declared statically.

### Module Creation Phase

Creation of the module object -- that is, the implementation of `ExecutionLoader.create_module` -- is governed by the `Py_mod_create` slot.

#### The Py_mod_create slot

The `Py_mod_create` slot is used to support custom module subclasses. The value pointer must point to a function with the following signature:

```c
PyObject* (*PyModuleCreateFunction)(PyObject *spec, PyModuleDef *def)
```

The function receives a ModuleSpec instance, as defined in PEP 451, and the `PyModuleDef` structure. It should return a new module object, or set an error and return NULL.

This function is not responsible for setting import-related attributes specified in PEP 451 (such as `__name__` or `__loader__`) on the new module.

There is no requirement for the returned object to be an instance of `types.ModuleType`. Any type can be used, as long as it supports setting and getting attributes, including at least the import-related attributes. However, only `ModuleType` instances support module-specific functionality such as per-module state and processing of execution slots. If something other than a `ModuleType` subclass is returned, no execution slots may be defined; if any are, a `SystemError` is raised.

Note that when this function is called, the module's entry in `sys.modules` is not populated yet. Attempting to import the same module again (possibly transitively), may lead to an infinite loop. Extension authors are advised to keep `Py_mod_create` minimal, an in particular to not call user code from it.

Multiple `Py_mod_create` slots may not be specified. If they are, import will fail with `SystemError`.

If `Py_mod_create` is not specified, the import machinery will create a normal module object using `PyModule_New`. The name is taken from *spec*.

#### Post-creation steps

If the `Py_mod_create` function returns an instance of `types.ModuleType` or a subclass (or if a `Py_mod_create` slot is not present), the import machinery will associate the `PyModuleDef` with the module. This also makes the `PyModuleDef` accessible to execution phase, the `PyModule_GetDef` function, and garbage collection routines (traverse, clear, free).

If the `Py_mod_create` function does not return a module subclass, then *m_size* must be 0, and *m_traverse*, *m_clear* and *m_free* must all be NULL. Otherwise, `SystemError` is raised.

Additionally, initial attributes specified in the `PyModuleDef` are set on the module object, regardless of its type:

* The docstring is set from *m_doc*, if non-NULL.
* The module's functions are initialized from *m_methods*, if any.

### Module Execution Phase

Module execution -- that is, the implementation of `ExecutionLoader.exec_module` -- is governed by "execution slots". This PEP only adds one, `Py_mod_exec`, but others may be added in the future.

The execution phase is done on the `PyModuleDef` associated with the module object. For objects that are not a subclass of `PyModule_Type` (for which `PyModule_GetDef` would fail), the execution phase is skipped.

Execution slots may be specified multiple times, and are processed in the order they appear in the slots array. When using the default import machinery, they are processed after import-related attributes specified in PEP 451 (such as `__name__` or `__loader__`) are set and the module is added to `sys.modules`.

#### Pre-Execution steps

Before processing the execution slots, per-module state is allocated for the module. From this point on, per-module state is accessible through `PyModule_GetState`.

#### The Py_mod_exec slot

The entry in this slot must point to a function with the following signature:

```c
int (*PyModuleExecFunction)(PyObject* module)
```

It will be called to initialize a module. Usually, this amounts to setting the module's initial attributes. The "module" argument receives the module object to initialize.

The function must return `0` on success, or, on error, set an exception and return `-1`.

### Legacy Init

The backwards-compatible single-phase initialization continues to be supported. In this scheme, the `PyInit` function returns a fully initialized module rather than a `PyModuleDef` object. In this case, the `PyInit` hook implements the creation phase, and the execution phase is a no-op.

Modules that need to work unchanged on older versions of Python should stick to single-phase initialization, because the benefits it brings can't be back-ported.

### Functions incompatible with multi-phase initialization

The `PyModule_Create` function will fail when used on a `PyModuleDef` structure with a non-NULL *m_slots* pointer. The function doesn't have access to the ModuleSpec object necessary for multi-phase initialization.

The `PyState_FindModule` function will return NULL, and `PyState_AddModule` and `PyState_RemoveModule` will also fail on modules with non-NULL *m_slots*. PyState registration is disabled because multiple module objects may be created from the same `PyModuleDef`.

### Export Hook Name

As portable C identifiers are limited to ASCII, module names must be encoded to form the PyInit hook name.

For ASCII module names, the import hook is named `PyInit_<modulename>`, where `<modulename>` is the name of the module.

For module names containing non-ASCII characters, the import hook is named `PyInitU_<encodedname>`, where the name is encoded using CPython's "punycode" encoding (RFC 3492 with a lowercase suffix), with hyphens ("-") replaced by underscores ("_").

In Python:

```python
def export_hook_name(name):
    try:
        suffix = b'_' + name.encode('ascii')
    except UnicodeEncodeError:
        suffix = b'U_' + name.encode('punycode').replace(b'-', b'_')
    return b'PyInit' + suffix
```

Examples:

| Module name | Init hook name |
|---|---|
| spam | `PyInit_spam` |
| lančmít | `PyInitU_lanmt_2sa6t` |
| スパム | `PyInitU_zck5b2b` |

For modules with non-ASCII names, single-phase initialization is not supported.

### Summary of API Changes and Additions

New functions:

* `PyModule_FromDefAndSpec` (macro)
* `PyModule_FromDefAndSpec2`
* `PyModule_ExecDef`
* `PyModule_SetDocString`
* `PyModule_AddFunctions`
* `PyModuleDef_Init`

New macros:

* `Py_mod_create`
* `Py_mod_exec`

New types:

* `PyModuleDef_Type` will be exposed

New structures:

* `PyModuleDef_Slot`

Other changes:

`PyModuleDef.m_reload` changes to `PyModuleDef.m_slots`.

`BuiltinImporter` and `ExtensionFileLoader` will now implement `create_module` and `exec_module`.

The internal `_imp` module will have backwards incompatible changes: `create_builtin`, `create_dynamic`, and `exec_dynamic` will be added; `init_builtin`, `load_dynamic` will be removed.

PEP 489 introduced multi-phase initialization and the `PyModuleDef_Slot` array that PEP 793 builds upon. PEP 793's "Rationale" defers to PEP 489's motivation for why multi-phase init exists, and its "Specification" reuses the same `PyModuleDef_Slot` structure -- the new `PyModExport_*` hook returns an array of these slots directly, without the wrapping `PyModuleDef` introduced here. PEP 793's `Py_mod_name`, `Py_mod_doc`, `Py_mod_state_size`, `Py_mod_methods`, `Py_mod_state_traverse`, `Py_mod_state_clear`, and `Py_mod_state_free` slots replace the `PyModuleDef` fields specified above. The "Rejected Ideas" section's note that a `PyModuleExport` hook was previously considered (with the comment "Keeping only the `PyInit` hook name ... yielded a much simpler solution") is the historical antecedent PEP 793 explicitly reverses.

*Source: https://peps.python.org/pep-0489/*

## Linked PEP 387 — Backwards Compatibility Policy
### Soft Deprecation

A soft deprecation can be used when using an API which should no longer be used to write new code, but it remains safe to continue using it in existing code. The API remains documented and tested, but will not be developed further (no enhancement).

The main difference between a "soft" and a (regular) "hard" deprecation is that the soft deprecation does not imply scheduling the removal of the deprecated API.

Another difference is that a soft deprecation does not issue a warning: it's only mentioned in the documentation, whereas usually a "hard" deprecation issues a `DeprecationWarning` warning at runtime. The documentation of a soft deprecation should explain why the API should be avoided, and if possible propose a replacement.

If the decision is made to deprecate (in the regular sense) a feature that is currently soft deprecated, the deprecation must follow the Backwards Compatibility Rules (i.e., there is no exception because the feature is already soft deprecated).

PEP 793's "Soft-deprecating the existing export hook" section invokes this definition: the existing `PyInit_*` API is marked as soft-deprecated, meaning it continues to work without warnings and remains fully supported and documented, but Python will not add new features to it. New features (such as combined free-threaded/non-free-threaded build support) only land on the new `PyModExport_*` hook.

*Source: https://peps.python.org/pep-0387/#soft-deprecation*