Single-phase initialization¶

The module initialization function may create and return the module object directly. This is referred to as “single-phase initialization”, and uses one of the following two module creation functions:

PyObject *PyModule_Create(PyModuleDef *def)¶

Return value: New reference.

Create a new module object, given the definition in def. This behaves like PyModule_Create2() with module_api_version set to PYTHON_API_VERSION.

PyObject *PyModule_Create2(PyModuleDef *def, int module_api_version)¶

Return value: New reference. Part of the Stable ABI.

Create a new module object, given the definition in def, assuming the API version module_api_version. If that version does not match the version of the running interpreter, a RuntimeWarning is emitted.

Return NULL with an exception set on error.

Note

Most uses of this function should be using PyModule_Create() instead; only use this if you are sure you need it.

Before it is returned from in the initialization function, the resulting module object is typically populated using functions like PyModule_AddObjectRef().

Multi-phase initialization¶

An alternate way to specify extensions is to request “multi-phase initialization”. Extension modules created this way behave more like Python modules: the initialization is split between the creation phase, when the module object is created, and the execution phase, when it is populated. The distinction is similar to the __new__() and __init__() methods of classes.

Unlike modules created using single-phase initialization, these modules are not singletons. For example, if the sys.modules entry is removed and the module is re-imported, a new module object is created, and typically populated with fresh method and type objects. The old module is subject to normal garbage collection. This mirrors the behavior of pure-Python modules.

Additional module instances may be created in sub-interpreters or after after Python runtime reinitialization (Py_Finalize() and Py_Initialize()). In these cases, sharing Python objects between module instances would likely cause crashes or undefined behavior.

To avoid such issues, each instance of an extension module should be isolated: changes to one instance should not implicitly affect the others, and all state, including references to Python objects, should be specific to a particular module instance. See Isolating Extension Modules for more details and a practical guide.

A simpler way to avoid these issues is raising an error on repeated initialization.

All modules created using multi-phase initialization are expected to support sub-interpreters, or otherwise explicitly signal a lack of support. This is usually achieved by isolation or blocking repeated initialization, as above. A module may also be limited to the main interpreter using the Py_mod_multiple_interpreters slot.

To request multi-phase initialization, the initialization function (PyInit_modulename) returns a PyModuleDef instance with non-empty m_slots. Before it is returned, the PyModuleDef instance must be initialized with the following function:

PyObject *PyModuleDef_Init(PyModuleDef *def)¶

Return value: Borrowed reference. Part of the Stable ABI since version 3.5.

Ensures a module definition is a properly initialized Python object that correctly reports its type and reference count.

Returns def cast to PyObject*, or NULL if an error occurred.

Added in version 3.5.

The m_slots member of the module definition must point to an array of PyModuleDef_Slot structures:

type PyModuleDef_Slot¶

Part of the Stable ABI (including all members) since version 3.5.

int slot¶

A slot ID, chosen from the available values explained below.

void *value¶

Value of the slot, whose meaning depends on the slot ID.

Added in version 3.5.

The m_slots array must be terminated by a slot with id 0.

The available slot types are:

Py_mod_create¶

Part of the Stable ABI since version 3.5.

Specifies a function that is called to create the module object itself. The value pointer of this slot must point to a function of the signature:

PyObject *create_module(PyObject *spec, PyModuleDef *def)¶

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

This function should be kept minimal. In particular, it should not call arbitrary Python code, as trying to import the same module again may result in an infinite loop.

Multiple Py_mod_create slots may not be specified in one module definition.

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, not the definition, to allow extension modules to dynamically adjust to their place in the module hierarchy and be imported under different names through symlinks, all while sharing a single module definition.

There is no requirement for the returned object to be an instance of PyModule_Type. Any type can be used, as long as it supports setting and getting import-related attributes. However, only PyModule_Type instances may be returned if the PyModuleDef has non-NULL m_traverse, m_clear, m_free; non-zero m_size; or slots other than Py_mod_create.

Added in version 3.5.

Py_mod_exec¶

Part of the Stable ABI since version 3.5.

Specifies a function that is called to execute the module. This is equivalent to executing the code of a Python module: typically, this function adds classes and constants to the module. The signature of the function is:

int exec_module(PyObject *module)¶

If multiple Py_mod_exec slots are specified, they are processed in the order they appear in the m_slots array.

Added in version 3.5.

Py_mod_multiple_interpreters¶

Part of the Stable ABI since version 3.12.

Specifies one of the following values:

Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED¶

The module does not support being imported in subinterpreters.

Py_MOD_MULTIPLE_INTERPRETERS_SUPPORTED¶

The module supports being imported in subinterpreters, but only when they share the main interpreter’s GIL. (See Isolating Extension Modules.)

Py_MOD_PER_INTERPRETER_GIL_SUPPORTED¶

The module supports being imported in subinterpreters, even when they have their own GIL. (See Isolating Extension Modules.)

This slot determines whether or not importing this module in a subinterpreter will fail.

Multiple Py_mod_multiple_interpreters slots may not be specified in one module definition.

If Py_mod_multiple_interpreters is not specified, the import machinery defaults to Py_MOD_MULTIPLE_INTERPRETERS_SUPPORTED.

Added in version 3.12.

Py_mod_gil¶

Part of the Stable ABI since version 3.13.

Specifies one of the following values:

Py_MOD_GIL_USED¶

The module depends on the presence of the global interpreter lock (GIL), and may access global state without synchronization.

Py_MOD_GIL_NOT_USED¶

The module is safe to run without an active GIL.

This slot is ignored by Python builds not configured with --disable-gil. Otherwise, it determines whether or not importing this module will cause the GIL to be automatically enabled. See Free-threaded CPython for more detail.

Multiple Py_mod_gil slots may not be specified in one module definition.

If Py_mod_gil is not specified, the import machinery defaults to Py_MOD_GIL_USED.

Added in version 3.13.

See PEP 489 for more details on multi-phase initialization.

Low-level module creation functions¶

The following functions are called under the hood when using multi-phase initialization. They can be used directly, for example when creating module objects dynamically. Note that both PyModule_FromDefAndSpec and PyModule_ExecDef must be called to fully initialize a module.

PyObject *PyModule_FromDefAndSpec(PyModuleDef *def, PyObject *spec)¶

Return value: New reference.

Create a new module object, given the definition in def and the ModuleSpec spec. This behaves like PyModule_FromDefAndSpec2() with module_api_version set to PYTHON_API_VERSION.

Added in version 3.5.

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

Return value: New reference. Part of the Stable ABI since version 3.7.

Create a new module object, given the definition in def and the ModuleSpec spec, assuming the API version module_api_version. If that version does not match the version of the running interpreter, a RuntimeWarning is emitted.

Return NULL with an exception set on error.

Note

Most uses of this function should be using PyModule_FromDefAndSpec() instead; only use this if you are sure you need it.

Added in version 3.5.

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

Part of the Stable ABI since version 3.7.

Process any execution slots (Py_mod_exec) given in def.

Added in version 3.5.

int PyModule_SetDocString(PyObject *module, const char *docstring)¶

Part of the Stable ABI since version 3.7.

Set the docstring for module to docstring. This function is called automatically when creating a module from PyModuleDef, using either PyModule_Create or PyModule_FromDefAndSpec.

Return 0 on success. Return -1 with an exception set on error.

Added in version 3.5.

int PyModule_AddFunctions(PyObject *module, PyMethodDef *functions)¶

Part of the Stable ABI since version 3.7.

Add the functions from the NULL terminated functions array to module. Refer to the PyMethodDef documentation for details on individual entries (due to the lack of a shared module namespace, module level “functions” implemented in C typically receive the module as their first parameter, making them similar to instance methods on Python classes). This function is called automatically when creating a module from PyModuleDef, using either PyModule_Create or PyModule_FromDefAndSpec.

The functions array must be statically allocated (or otherwise guaranteed to outlive the module object).

Added in version 3.5.

Support functions¶

The module initialization function (if using single phase initialization) or a function called from a module execution slot (if using multi-phase initialization), can use the following functions to help initialize the module state:

int PyModule_AddObjectRef(PyObject *module, const char *name, PyObject *value)¶

Part of the Stable ABI since version 3.10.

Add an object to module as name. This is a convenience function which can be used from the module’s initialization function.

On success, return 0. On error, raise an exception and return -1.

Example usage:

static int
add_spam(PyObject *module, int value)
{
    PyObject *obj = PyLong_FromLong(value);
    if (obj == NULL) {
        return -1;
    }
    int res = PyModule_AddObjectRef(module, "spam", obj);
    Py_DECREF(obj);
    return res;
 }

To be convenient, the function accepts NULL value with an exception set. In this case, return -1 and just leave the raised exception unchanged.

The example can also be written without checking explicitly if obj is NULL:

static int
add_spam(PyObject *module, int value)
{
    PyObject *obj = PyLong_FromLong(value);
    int res = PyModule_AddObjectRef(module, "spam", obj);
    Py_XDECREF(obj);
    return res;
 }

Note that Py_XDECREF() should be used instead of Py_DECREF() in this case, since obj can be NULL.

The number of different name strings passed to this function should be kept small, usually by only using statically allocated strings as name. For names that aren’t known at compile time, prefer calling PyUnicode_FromString() and PyObject_SetAttr() directly. For more details, see PyUnicode_InternFromString(), which may be used internally to create a key object.

Added in version 3.10.

int PyModule_Add(PyObject *module, const char *name, PyObject *value)¶

Part of the Stable ABI since version 3.13.

Similar to PyModule_AddObjectRef(), but “steals” a reference to value (even on error). It can be called with a result of function that returns a new reference without bothering to check its result or even saving it to a variable.

Example usage:

if (PyModule_Add(module, "spam", PyBytes_FromString(value)) < 0) {
    goto error;
}

Added in version 3.13.

int PyModule_AddObject(PyObject *module, const char *name, PyObject *value)¶

Part of the Stable ABI.

Similar to PyModule_AddObjectRef(), but steals a reference to value on success (if it returns 0).

The new PyModule_Add() or PyModule_AddObjectRef() functions are recommended, since it is easy to introduce reference leaks by misusing the PyModule_AddObject() function.

Note

Unlike other functions that steal references, PyModule_AddObject() only releases the reference to value on success.

This means that its return value must be checked, and calling code must Py_XDECREF() value manually on error.

Example usage:

PyObject *obj = PyBytes_FromString(value);
if (PyModule_AddObject(module, "spam", obj) < 0) {
    // If 'obj' is not NULL and PyModule_AddObject() failed,
    // 'obj' strong reference must be deleted with Py_XDECREF().
    // If 'obj' is NULL, Py_XDECREF() does nothing.
    Py_XDECREF(obj);
    goto error;
}
// PyModule_AddObject() stole a reference to obj:
// Py_XDECREF(obj) is not needed here.

Soft deprecated since version 3.13.

int PyModule_AddIntConstant(PyObject *module, const char *name, long value)¶

Part of the Stable ABI.

Add an integer constant to module as name. This convenience function can be used from the module’s initialization function. Return -1 with an exception set on error, 0 on success.

This is a convenience function that calls PyLong_FromLong() and PyModule_AddObjectRef(); see their documentation for details.

int PyModule_AddStringConstant(PyObject *module, const char *name, const char *value)¶

Part of the Stable ABI.

Add a string constant to module as name. This convenience function can be used from the module’s initialization function. The string value must be NULL-terminated. Return -1 with an exception set on error, 0 on success.

This is a convenience function that calls PyUnicode_InternFromString() and PyModule_AddObjectRef(); see their documentation for details.

PyModule_AddIntMacro(module, macro)¶

Add an int constant to module. The name and the value are taken from macro. For example PyModule_AddIntMacro(module, AF_INET) adds the int constant AF_INET with the value of AF_INET to module. Return -1 with an exception set on error, 0 on success.

PyModule_AddStringMacro(module, macro)¶

Add a string constant to module.

int PyModule_AddType(PyObject *module, PyTypeObject *type)¶

Part of the Stable ABI since version 3.10.

Add a type object to module. The type object is finalized by calling internally PyType_Ready(). The name of the type object is taken from the last component of tp_name after dot. Return -1 with an exception set on error, 0 on success.

Added in version 3.9.

int PyUnstable_Module_SetGIL(PyObject *module, void *gil)¶

This is Unstable API. It may change without warning in minor releases.

Indicate that module does or does not support running without the global interpreter lock (GIL), using one of the values from Py_mod_gil. It must be called during module’s initialization function. If this function is not called during module initialization, the import machinery assumes the module does not support running without the GIL. This function is only available in Python builds configured with --disable-gil. Return -1 with an exception set on error, 0 on success.

Added in version 3.13.