python-peps/pep-0489.txt

PEP: 489
Title: Redesigning extension module loading
Version: $Revision$
Last-Modified: $Date$
Author: Petr Viktorin <encukou@gmail.com>,
        Stefan Behnel <stefan_ml@behnel.de>,
        Nick Coghlan <ncoghlan@gmail.com>
Discussions-To: import-sig@python.org
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 11-Aug-2013
Python-Version: 3.5
Post-History: 23-Aug-2013, 20-Feb-2015, 16-Apr-2015
Resolution:


Abstract
========

This PEP proposes a redesign of the way in which 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 them
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.


Motivation
==========

Python modules and extension modules are not being set up in the same way.
For Python modules, the module 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, 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 the FQMN, 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 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 either NULL in
the case of an exception, or 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 struct. 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 current extension module initialization will be deprecated in favor of
a new initialization scheme. Since the current scheme will continue to be
available, existing code will continue to work unchanged, including binary
compatibility.

Extension modules that support the new initialization scheme must export
the public symbol "PyModuleExport_<modulename>", where "modulename"
is the name of the module. (For modules with non-ASCII names the symbol name
is slightly different, see "Export Hook Name" below.)

If defined, this symbol must resolve to a C function with the following
signature::

    PyModuleDef* (*PyModuleExportFunction)(void)

For cross-platform compatibility, the function should be declared as::

    PyMODEXPORT_FUNC PyModuleExport_<modulename>(void)

The function must return a pointer to a PyModuleDef structure.
This structure must be available for the lifetime of the module created from
it – usually, it will be declared statically.

Alternatively, this function can return NULL, in which case it is as if the
symbol was not defined – see the "Legacy Init" section.

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::

    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 the new import mechanism, m_size must not be negative.
Also, the *m_name* field of PyModuleDef will not be unused during importing;
the module name will be taken from the ModuleSpec.


Module Creation
---------------

Module creation – 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::

    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 [#pep-0451-attributes]_ (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.

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 by PyModule_New. The name is taken from *spec*.


Post-creation steps
...................

If the Py_mod_create function returns an instance of types.ModuleType
(or subclass), or if a Py_mod_create slot is not present, the import machinery
will do the following steps after the module is created:

* If *m_size* is specified, per-module state is allocated and made accessible
  through PyModule_GetState
* The PyModuleDef is associated with the module, making it accessible to
  PyModule_GetDef, and enabling the m_traverse, m_clear and m_free hooks.
* The docstring is set from m_doc.
* The module's functions are initialized from m_methods.

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


Module Execution
----------------

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.

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 [#pep-0451-attributes]_
(such as ``__name__`` or ``__loader__``) are set and the module is added
to sys.modules.


The Py_mod_exec slot
....................

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

    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.

If PyModuleExec replaces the module's entry in sys.modules,
the new object will be used and returned by importlib machinery.
(This mirrors the behavior of Python modules. Note that for extensions,
implementing Py_mod_create is usually a better solution for the use cases
this serves.)

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


Legacy Init
-----------

If the PyModuleExport function is not defined, or if it returns NULL, the
import machinery will try to initialize the module using the
"PyInit_<modulename>" hook, as described in PEP 3121.

If the PyModuleExport function is defined, the PyInit function will be ignored.
Modules requiring compatibility with previous versions of CPython may implement
the PyInit function in addition to the new hook.

Modules using the legacy init API will be initialized entirely in the
Loader.create_module step; Loader.exec_module will be a no-op.

A module that supports older CPython versions can be coded as::

    #define Py_LIMITED_API
    #include <Python.h>

    static int spam_exec(PyObject *module) {
        PyModule_AddStringConstant(module, "food", "spam");
        return 0;
    }

    static PyModuleDef_Slot spam_slots[] = {
        {Py_mod_exec, spam_exec},
        {0, NULL}
    };

    static PyModuleDef spam_def = {
        PyModuleDef_HEAD_INIT,                      /* m_base */
        "spam",                                     /* m_name */
        PyDoc_STR("Utilities for cooking spam"),    /* m_doc */
        0,                                          /* m_size */
        NULL,                                       /* m_methods */
        spam_slots,                                 /* m_slots */
        NULL,                                       /* m_traverse */
        NULL,                                       /* m_clear */
        NULL,                                       /* m_free */
    };

    PyModuleDef* PyModuleExport_spam(void) {
        return &spam_def;
    }

    PyMODINIT_FUNC
    PyInit_spam(void) {
        PyObject *module;
        module = PyModule_Create(&spam_def);
        if (module == NULL) return NULL;
        if (spam_exec(module) != 0) {
            Py_DECREF(module);
            return NULL;
        }
        return module;
    }

Note that this must be *compiled* on a new CPython version, but the resulting
shared library will be backwards compatible.
(Source-level compatibility is possible with preprocessor directives.)

If a Py_mod_create slot is used, PyInit should call its function instead of
PyModule_Create. Keep in mind that the ModuleSpec object is not available in
the legacy init scheme.


Subinterpreters and Interpreter Reloading
-----------------------------------------

Extensions using the new initialization scheme are expected to support
subinterpreters and multiple Py_Initialize/Py_Finalize cycles correctly.
The mechanism is designed to make this easy, but care is still required
on the part of the extension author.
No user-defined functions, methods, or instances may leak to different
interpreters.
To achieve this, all module-level state should be kept in either the module
dict, or in the module object's storage reachable by PyModule_GetState.
A simple rule of thumb is: Do not define any static data, except built-in types
with no mutable or user-settable class attributes.

Behavior of existing module creation functions
----------------------------------------------

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
"new style" module creation.

The PyState_FindModule function will return NULL, and PyState_AddModule
and PyState_RemoveModule will fail with SystemError.
PyState registration is disabled because multiple module objects may be
created from the same PyModuleDef.


Module state and C-level callbacks
----------------------------------

Due to the unavailability of PyState_FindModule, any function that needs access
to module-level state (including functions, classes or exceptions defined at
the module level) must receive a reference to the module object (or the
particular object it needs), either directly or indirectly.
This is currently difficult in two situations:

* Methods of classes, which receive a reference to the class, but not to
  the class's module
* Libraries with C-level callbacks, unless the callbacks can receive custom
  data set at cllback registration

Fixing these cases is outside of the scope of this PEP, but will be needed for
the new mechanism to be useful to all modules. Proper fixes have been discussed
on the import-sig mailing list [#findmodule-discussion]_.

As a rule of thumb, modules that rely on PyState_FindModule are, at the moment,
not good candidates for porting to the new mechanism.


New Functions
-------------

A new function and macro will be added to implement module creation.
These are similar to PyModule_Create and PyModule_Create2, except they
take an additional ModuleSpec argument, and handle module definitions with
non-NULL slots::

    PyObject * PyModule_FromDefAndSpec(PyModuleDef *def, PyObject *spec)
    PyObject * PyModule_FromDefAndSpec2(PyModuleDef *def, PyObject *spec,
                                        int module_api_version)

A new function will be added to run "execution slots" on a module::

    PyAPI_FUNC(int) PyModule_ExecDef(PyObject *module, PyModuleDef *def)

Additionally, two helpers will be added for setting the docstring and
methods on a module::

    int PyModule_SetDocString(PyObject *, const char *)
    int PyModule_AddFunctions(PyObject *, PyMethodDef *)


Export Hook Name
----------------

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

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

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


In 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'PyModuleExport' + suffix

Examples:

=============  ===========================
Module name    Export hook name
=============  ===========================
spam           PyModuleExport_spam
lančmít        PyModuleExportU_lanmt_2sa6t
スパム          PyModuleExportU_zck5b2b
=============  ===========================


Module Reloading
----------------

Reloading an extension module using importlib.reload() will continue to
have no effect, except re-setting import-related attributes.

Due to limitations in shared library loading (both dlopen on POSIX and
LoadModuleEx on Windows), it is not generally possible to load
a modified library after it has changed on disk.

Use cases for reloading other than trying out a new version of the module
are too rare to require all module authors to keep reloading in mind.
If reload-like functionality is needed, authors can export a dedicated
function for it.


Multiple modules in one library
-------------------------------

To support multiple Python modules in one shared library, the library can
export additional PyModuleExport* symbols besides the one that corresponds
to the library's filename.

Note that this mechanism can currently only be used to *load* extra modules,
not to *find* them.

Given the filesystem location of a shared library and a module name,
a module may be loaded with::

    import importlib.machinery
    import importlib.util
    loader = importlib.machinery.ExtensionFileLoader(name, path)
    spec = importlib.util.spec_from_loader(name, loader)
    module = importlib.util.module_from_spec(spec)
    loader.exec_module(module)
    return module

On platforms that support symbolic links, these may be used to install one
library under multiple names, exposing all exported modules to normal
import machinery.


Testing and initial implementations
-----------------------------------

For testing, a new built-in module ``_testmoduleexport`` will be created.
The library will export several additional modules using the mechanism
described in "Multiple modules in one library".

The ``_testcapi`` module will be unchanged, and will use the old API
indefinitely (or until the old API is removed).

The ``array`` and ``xx*`` modules will be converted to the new API as
part of the initial implementation.


API Changes and Additions
-------------------------

New functions:

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

New macros:

* PyMODEXPORT_FUNC
* Py_mod_create
* Py_mod_exec

New structures:

* PyModuleDef_Slot

PyModuleDef.m_reload changes to PyModuleDef.m_slots.


Possible Future Extensions
==========================

The slots mechanism, inspired by PyType_Slot from PEP 384,
allows later extensions.

Some extension modules exports many constants; for example _ssl has
a long list of calls in the form::

    PyModule_AddIntConstant(m, "SSL_ERROR_ZERO_RETURN",
                            PY_SSL_ERROR_ZERO_RETURN);

Converting this to a declarative list, similar to PyMethodDef,
would reduce boilerplate, and provide free error-checking which
is often missing.

String constants and types can be handled similarly.
(Note that non-default bases for types cannot be portably specified
statically; this case would need a Py_mod_exec function that runs
before the slots are added. The free error-checking would still be
beneficial, though.)

Another possibility is providing a "main" function that would be run
when the module is given to Python's -m switch.
For this to work, the runpy module will need to be modified to take
advantage of ModuleSpec-based loading introduced in PEP 451.
Also, it will be necessary to add a mechanism for setting up a module
according to slots it wasn't originally defined with.


Implementation
==============

Work-in-progress implementation is available in a Github repository [#gh-repo]_;
a patchset is at [#gh-patch]_.


Previous Approaches
===================

Stefan Behnel's initial proto-PEP [#stefans_protopep]_
had a "PyInit_modulename" hook that would create a module class,
whose ``__init__`` would be then called to create the module.
This proposal did not correspond to the (then nonexistent) PEP 451,
where module creation and initialization is broken into distinct steps.
It also did not support loading an extension into pre-existing module objects.

Nick Coghlan proposed "Create" and "Exec" hooks, and wrote a prototype
implementation [#nicks-prototype]_.
At this time PEP 451 was still not implemented, so the prototype
does not use ModuleSpec.

The original version of this PEP used Create and Exec hooks, and allowed
loading into arbitrary pre-constructed objects with Exec hook.
The proposal made extension module initialization closer to how Python modules
are initialized, but it was later recognized that this isn't an important goal.
The current PEP describes a simpler solution.


References
==========

.. [#lazy_import_concerns]
   https://mail.python.org/pipermail/python-dev/2013-August/128129.html

.. [#pep-0451-attributes]
   https://www.python.org/dev/peps/pep-0451/#attributes

.. [#stefans_protopep]
   https://mail.python.org/pipermail/python-dev/2013-August/128087.html

.. [#nicks-prototype]
   https://mail.python.org/pipermail/python-dev/2013-August/128101.html

.. [#rfc-3492]
   http://tools.ietf.org/html/rfc3492

.. [#gh-repo]
   https://github.com/encukou/cpython/commits/pep489

.. [#gh-patch]
   https://github.com/encukou/cpython/compare/master...encukou:pep489.patch

.. [#findmodule-discussion]
   https://mail.python.org/pipermail/import-sig/2015-April/000959.html


Copyright
=========

This document has been placed in the public domain.