Module definition¶

Modules created using the C API are typically defined using an array of PySlot structs, which provides a “description” of how a module should be created. See Definition slots for more information on slots in general.

The slots array is usually used to define an extension module’s “main” module object (see Defining extension modules for details). It can also be used to create extension modules dynamically.

Unless specified otherwise, the same slot ID may not be repeated in an array of slots.

Module state¶

Extension modules can have module state – a piece of memory that is allocated on module creation, and freed when the module object is deallocated. The module state is specified using dedicated slots.

A typical use of module state is storing an exception type – or indeed any type object defined by the module –

Unlike the module’s Python attributes, Python code cannot replace or delete data stored in module state.

Keeping per-module information in attributes and module state, rather than in static globals, makes module objects isolated and safer for use in multiple sub-interpreters. It also helps Python do an orderly clean-up when it shuts down.

Extensions that keep references to Python objects as part of module state must implement Py_mod_state_traverse and Py_mod_state_clear functions to avoid reference leaks.

To retrieve the state from a given module, use the following functions:

void *PyModule_GetState(PyObject *module)¶

Part of the Stable ABI.

Return the “state” of the module, that is, a pointer to the block of memory allocated at module creation time, or NULL. See Py_mod_state_size.

On error, return NULL with an exception set. Use PyErr_Occurred() to tell this case apart from missing module state.

int PyModule_GetStateSize(PyObject *module, Py_ssize_t *result)¶

Part of the Stable ABI since version 3.15.

Set *result to the size of module’s state, as specified using Py_mod_state_size (or PyModuleDef.m_size), and return 0.

On error, set *result to -1, and return -1 with an exception set.

Added in version 3.15.

PyObject *module = <the module in question>;

void *module_token;
if (PyModule_GetToken(module, &module_token) < 0) {
    return NULL;
}
if (module_token != your_token) {
    PyErr_SetString(PyExc_ValueError, "unexpected module");
    return NULL;
}

// This module's state has the expected memory layout; it's safe to cast
struct my_state *state = (struct my_state*)PyModule_GetState(module);

Creating extension modules dynamically¶

The following functions may be used to create an extension module dynamically, rather than from an extension’s export hook.

PyObject *PyModule_FromSlotsAndSpec(const PySlot *slots, PyObject *spec)¶

Return value: New reference. Part of the Stable ABI since version 3.15.

Create a new module object, given an array of slots and the ModuleSpec spec.

The slots argument must point to an array of PySlot structures, terminated by an entry with slot ID of 0 (typically written as PySlot_END). The array must include a Py_mod_abi entry.

The spec argument may be any ModuleSpec-like object, as described in Py_mod_create documentation. Currently, the spec must have a name attribute.

On success, return the new module. On error, return NULL with an exception set.

Note that this does not process the module’s execution slot (Py_mod_exec). Both PyModule_FromSlotsAndSpec() and PyModule_Exec() must be called to fully initialize a module. (See also Multi-phase initialization.)

Added in version 3.15.

int PyModule_Exec(PyObject *module)¶

Part of the Stable ABI since version 3.15.

Execute the Py_mod_exec slot(s) of module.

On success, return 0. On error, return -1 with an exception set.

For clarity: If module has no slots, for example if it uses legacy single-phase initialization, this function does nothing and returns 0.

Added in version 3.15.

Module definition struct¶

Traditionally, extension modules were defined using a module definition as the “description” of how a module should be created. Rather than using an array of slots directly, the definition has dedicated members for most common functionality, and allows additional slots as an extension mechanism.

This way of defining modules is still available and there are no plans to remove it.

type PyModuleDef¶

Part of the Stable ABI (see below).

The module definition struct, which holds information needed to create a module object.

This structure must be statically allocated (or be otherwise guaranteed to be valid while any modules created from it exist). Usually, there is only one variable of this type for each extension module defined this way.

The struct, including all members, is part of the Stable ABI for non-free-threaded builds (abi3). In the Stable ABI for free-threaded builds (abi3t), this struct is opaque, and unusable in practice; see Module definition for a replacement.

PyModuleDef_Base m_base¶

Always initialize this member to PyModuleDef_HEAD_INIT:

type PyModuleDef_Base¶

Part of the Stable ABI (see below).

The type of PyModuleDef.m_base.

The struct is part of the Stable ABI for non-free-threaded builds (abi3). In the Stable ABI for Free-Threaded Builds (abi3t), this struct is opaque, and unusable in practice.

PyModuleDef_HEAD_INIT¶

The required initial value for PyModuleDef.m_base.

const char *m_name¶

Corresponds to the Py_mod_name slot.

const char *m_doc¶

These members correspond to the Py_mod_doc slot. Setting this to NULL is equivalent to omitting the slot.

Py_ssize_t m_size¶

Corresponds to the Py_mod_state_size slot. Setting this to zero is equivalent to omitting the slot.

When using legacy single-phase initialization or when creating modules dynamically using PyModule_Create() or PyModule_Create2(), m_size may be set to -1. This indicates that the module does not support sub-interpreters, because it has global state.

PyMethodDef *m_methods¶

Corresponds to the Py_mod_methods slot. Setting this to NULL is equivalent to omitting the slot.

PyModuleDef_Slot *m_slots¶

An array of additional slots, terminated by a {0, NULL} entry. Note that the entries use the older PyModuleDef_Slot structure, rather than PySlot.

If the array contains slots corresponding to PyModuleDef members, the values must match. For example, if you use Py_mod_name in m_slots, PyModuleDef.m_name must be set to the same pointer (not just an equal string).

Changed in version 3.5: Prior to version 3.5, this member was always set to NULL, and was defined as:

inquiry m_reload¶

type PyModuleDef_Slot¶

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

Older structure defining additional slots of a module.

Note that a PyModuleDef_Slot array may be included in a PySlot array using Py_mod_slots, and vice versa using Py_slot_subslots.

Each PyModuleDef_Slot structure modslot is interpreted as the following PySlot structure:

(PySlot){
   .sl_id=modslot.slot,
   .sl_flags=PySlot_INTPTR | sub_static,
   .sl_ptr=modslot.value
}

where sub_static is PySlot_STATIC if the slot requires the flag (such as for Py_mod_methods), or if this flag is present on the “parent” Py_mod_slots slot (if any).

int slot¶

Corresponds to PySlot.sl_id.

void *value¶

Corresponds to PySlot.sl_ptr.

Added in version 3.5.

traverseproc m_traverse¶

inquiry m_clear¶

freefunc m_free¶

These members correspond to the Py_mod_state_traverse, Py_mod_state_clear, and Py_mod_state_free slots, respectively.

Setting these members to NULL is equivalent to omitting the corresponding slots.

Changed in version 3.9: m_traverse, m_clear and m_free functions are no longer called before the module state is allocated.

PyTypeObject PyModuleDef_Type¶

Part of the Stable ABI since version 3.5.

The type of PyModuleDef objects.

Py_mod_slots¶

Part of the Stable ABI since version 3.15.

Slot ID that works like Py_slot_subslots, except it specifies an array of PyModuleDef_Slot structures.

Added in version 3.15.

The following API can be used to create modules from a PyModuleDef struct:

PyObject *PyModule_Create(PyModuleDef *def)¶

Return value: New reference.

Create a new module object, given the definition in def. This is a macro that calls PyModule_Create2() with module_api_version set to PYTHON_API_VERSION, or to PYTHON_ABI_VERSION if using the limited API.

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.

This function does not support slots. The m_slots member of def must be NULL.

Note

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

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

Return value: New reference.

This macro calls PyModule_FromDefAndSpec2() with module_api_version set to PYTHON_API_VERSION, or to PYTHON_ABI_VERSION if using the limited API.

Added in version 3.5.

Soft deprecated since version 3.15: Prefer PyModule_FromSlotsAndSpec() in new code.

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 that this does not process execution slots (Py_mod_exec). Both PyModule_FromDefAndSpec and PyModule_ExecDef must be called to fully initialize a module.

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.

Soft deprecated since version 3.15: Prefer PyModule_FromSlotsAndSpec() in new code.

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.

Soft deprecated since version 3.15: To run a module’s own execution slots, prefer PyModule_Exec(), which works on modules that were not created from a PyModuleDef structure.

PYTHON_API_VERSION¶

PYTHON_API_STRING¶

The C API version, as an integer (1013) and string ("1013"), respectively. Defined for backwards compatibility.

Currently, this constant is not updated in new Python versions, and is not useful for versioning. This may change in the future.

PYTHON_ABI_VERSION¶

PYTHON_ABI_STRING¶

Defined as 3 and "3", respectively, for backwards compatibility.

Currently, this constant is not updated in new Python versions, and is not useful for versioning. This may change in the future.

Support functions¶

The following functions are provided to help initialize a module object. They are intended for a module’s execution slot (Py_mod_exec), the initialization function for legacy single-phase initialization, or code that creates modules dynamically.

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 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 (such as when using Multi-phase initialization, PyModule_Create, or PyModule_FromDefAndSpec). Some module authors may prefer defining functions in multiple PyMethodDef arrays; in that case they should call this function directly.

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

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 (such as when using Multi-phase initialization, PyModule_Create, or PyModule_FromDefAndSpec).

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

Added in version 3.5.

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 when using Legacy single-phase initialization. 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.