# Capsules

Refer to Providing a C API for an Extension Module for more information on using these objects.

Added in version 3.1.

type PyCapsule

  This subtype of `PyObject` represents an opaque value, useful for C extension modules which need to pass an opaque value (as a void* pointer) through Python code to other C code. It is often used to make a C function pointer defined in one module available to other modules, so the regular import mechanism can be used to access C APIs defined in dynamically loaded modules.

PyTypeObject PyCapsule_Type
  Part of the Stable ABI.

  The type object corresponding to capsule objects. This is the same object as `types.CapsuleType` in the Python layer.

type PyCapsule_Destructor
  Part of the Stable ABI.

  The type of a destructor callback for a capsule. Defined as:

      typedef void (*PyCapsule_Destructor)(PyObject *);

  See `PyCapsule_New()` for the semantics of PyCapsule_Destructor callbacks.

int PyCapsule_CheckExact(PyObject *p)
  Thread safety: Atomic.

  Return true if its argument is a `PyCapsule`. This function always succeeds.

PyObject *PyCapsule_New(void *pointer, const char *name, PyCapsule_Destructor destructor)
  Return value: New reference. Part of the Stable ABI. Thread safety: Atomic.

  Create a `PyCapsule` encapsulating the pointer. The pointer argument may not be `NULL`.

  On failure, set an exception and return `NULL`.

  The name string may either be `NULL` or a pointer to a valid C string. If non-`NULL`, this string must outlive the capsule. (Though it is permitted to free it inside the destructor.)

  If the destructor argument is not `NULL`, it will be called with the capsule as its argument when it is destroyed.

  If this capsule will be stored as an attribute of a module, the name should be specified as `modulename.attributename`. This will enable other modules to import the capsule using `PyCapsule_Import()`.

void *PyCapsule_GetPointer(PyObject *capsule, const char *name)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Retrieve the pointer stored in the capsule. On failure, set an exception and return `NULL`.

  The name parameter must compare exactly to the name stored in the capsule. If the name stored in the capsule is `NULL`, the name passed in must also be `NULL`. Python uses the C function `strcmp()` to compare capsule names.

PyCapsule_Destructor PyCapsule_GetDestructor(PyObject *capsule)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Return the current destructor stored in the capsule. On failure, set an exception and return `NULL`.

  It is legal for a capsule to have a `NULL` destructor. This makes a `NULL` return code somewhat ambiguous; use `PyCapsule_IsValid()` or `PyErr_Occurred()` to disambiguate.

void *PyCapsule_GetContext(PyObject *capsule)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Return the current context stored in the capsule. On failure, set an exception and return `NULL`.

  It is legal for a capsule to have a `NULL` context. This makes a `NULL` return code somewhat ambiguous; use `PyCapsule_IsValid()` or `PyErr_Occurred()` to disambiguate.

const char *PyCapsule_GetName(PyObject *capsule)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Return the current name stored in the capsule. On failure, set an exception and return `NULL`.

  It is legal for a capsule to have a `NULL` name. This makes a `NULL` return code somewhat ambiguous; use `PyCapsule_IsValid()` or `PyErr_Occurred()` to disambiguate.

void *PyCapsule_Import(const char *name, int no_block)
  Part of the Stable ABI. Thread safety: Safe to call from multiple threads with external synchronization only.

  Import a pointer to a C object from a capsule attribute in a module. The name parameter should specify the full name to the attribute, as in `module.attribute`. The name stored in the capsule must match this string exactly.

  This function splits name on the `.` character, and imports the first element. It then processes further elements using attribute lookups.

  Return the capsule’s internal pointer on success. On failure, set an exception and return `NULL`.

  Note

  If name points to an attribute of some submodule or subpackage, this submodule or subpackage must be previously imported using other means (for example, by using `PyImport_ImportModule()`) for the attribute lookups to succeed.

  Changed in version 3.3: no_block has no effect anymore.

int PyCapsule_IsValid(PyObject *capsule, const char *name)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Determines whether or not capsule is a valid capsule. A valid capsule is non-`NULL`, passes `PyCapsule_CheckExact()`, has a non-`NULL` pointer stored in it, and its internal name matches the name parameter. (See `PyCapsule_GetPointer()` for information on how capsule names are compared.)

  In other words, if `PyCapsule_IsValid()` returns a true value, calls to any of the accessors (any function starting with `PyCapsule_Get`) are guaranteed to succeed.

  Return a nonzero value if the object is valid and matches the name passed in. Return `0` otherwise. This function will not fail.

int PyCapsule_SetContext(PyObject *capsule, void *context)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Set the context pointer inside capsule to context.

  Return `0` on success. Return nonzero and set an exception on failure.

int PyCapsule_SetDestructor(PyObject *capsule, PyCapsule_Destructor destructor)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Set the destructor inside capsule to destructor.

  Return `0` on success. Return nonzero and set an exception on failure.

int PyCapsule_SetName(PyObject *capsule, const char *name)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Set the name inside capsule to name. If non-`NULL`, the name must outlive the capsule. If the previous name stored in the capsule was not `NULL`, no attempt is made to free it.

  Return `0` on success. Return nonzero and set an exception on failure.

int PyCapsule_SetPointer(PyObject *capsule, void *pointer)
  Part of the Stable ABI. Thread safety: Safe to call without external synchronization on distinct objects.

  Set the void pointer inside capsule to pointer. The pointer may not be `NULL`.

  Return `0` on success. Return nonzero and set an exception on failure.