PEP 547 – Running extension modules using the -m option
- Marcel Plch <gmarcel.plch at gmail.com>, Petr Viktorin <encukou at gmail.com>
- Standards Track
Cython – the most important use case for this PEP and the only explicit one – is not ready for multi-phase initialization yet. It keeps global state in C-level static variables. See discussion at Cython issue 1923.
The PEP is deferred until the situation changes.
This PEP proposes implementation that allows built-in and extension
modules to be executed in the
__main__ namespace using
the PEP 489 multi-phase initialization.
With this, a multi-phase initialization enabled module can be run using following command:
$ python3 -m _testmultiphase This is a test module named __main__.
Currently, extension modules do not support all functionality of
Python source modules.
Specifically, it is not possible to run extension modules as scripts using
The technical groundwork to make this possible has been done for PEP 489,
and enabling the
-m option is listed in that PEP’s
“Possible Future Extensions” section.
Technically, the additional changes proposed here are relatively small.
Extension modules’ lack of support for the
-m option has traditionally
been worked around by providing a Python wrapper.
For example, the
_pickle module’s command line interface is in the
pickle module (along with a pure-Python reimplementation).
This works well for standard library modules, as building command line interfaces using the C API is cumbersome. However, other users may want to create executable extension modules directly.
An important use case is Cython, a Python-like language that compiles to
C extension modules.
Cython is a (near) superset of Python, meaning that compiling a Python module
with Cython will typically not change the module’s functionality, allowing
Cython-specific features to be added gradually.
This PEP will allow Cython extension modules to behave the same as their Python
counterparts when run using the
Cython developers consider the feature worth implementing (see
Cython issue 1715).
-m option is handled by the function
The module specified by
-m is not imported normally.
Instead, it is executed in the namespace of the
which is created quite early in interpreter initialization.
For Python source modules, running in another module’s namespace is not
a problem: the code is executed with
globals set to the
This is not the case for extension modules, whose
PyInit_* entry point
traditionally both created a new module object (using
and initialized it.
Since Python 3.5, extension modules can use PEP 489 multi-phase initialization.
In this scenario, the
PyInit_* entry point returns a
structure: a description of how the module should be created and initialized.
The extension can choose to customize creation of the module object using
Py_mod_create callback, or opt to use a normal module object by not
Py_mod_exec, is then called to initialize the module
object, e.g. by populating it with methods and classes.
Multi-phase initialization makes it possible to execute an extension module in
another module’s namespace: if a
Py_mod_create callback is not specified,
__main__ module can be passed to the
Py_mod_exec callback to be
initialized, as if
__main__ was a freshly constructed module object.
One complication in this scheme is C-level module state.
Each module has a
md_state pointer that points to a region of memory
allocated when an extension module is created.
PyModuleDef specifies how much memory is to be allocated.
The implementation must take care that
md_state memory is allocated at most
Py_mod_exec callback should only be called once per module.
The implications of multiply-initialized modules are too subtle to require
expecting extension authors to reason about them.
md_state pointer itself will serve as a guard: allocating the memory
Py_mod_exec will always be done together, and initializing an
extension module will fail if
md_state is already non-NULL.
__main__ module is not created as an extension module,
md_state is normally
Before initializing an extension module in
__main__’s context, its module
state will be allocated according to the
PyModuleDef of that module.
While PEP 489 was designed to make these changes generally possible,
it’s necessary to decouple module discovery, creation, and initialization
steps for extension modules, so that another module can be used instead of
a newly initialized one, and the functionality needs to be added to
A new optional method for importlib loaders will be added.
This method will be called
exec_in_module and will take two
positional arguments: module spec and an already existing module.
Any import-related attributes, such as
already set on the module will be ignored.
runpy._run_module_as_main function will look for this new
If it is present,
runpy will execute it instead of trying to load and
run the module’s Python code.
runpy will act as before.
ExtensionFileLoader will get an implementation of
exec_in_module that will call a new function,
_imp.exec_in_module will use existing machinery to find and call an
PyInit_* function can return either a fully initialized module
(single-phase initialization) or a
PyModuleDef (for PEP 489 multi-phase
In the single-phase initialization case,
_imp.exec_in_module will raise
In the multi-phase initialization case, the
PyModuleDef and the module to
be initialized will be passed to a new function,
This function raises
ImportError if the
Py_mod_create slot, or if the module has already been initialized
md_state pointer is not
Otherwise, the function will initialize the module according to the
This PEP maintains backwards compatibility.
It only adds new functions, and a new loader method that is added for
a loader that previously did not support running modules as
The reference implementation of this PEP is available at GitHub.
This document has been placed in the public domain.
Last modified: 2023-09-09 17:39:29 GMT