Backported PyCapsule from 3.1, and converted most uses of
CObject to PyCapsule.
diff --git a/Doc/c-api/capsule.rst b/Doc/c-api/capsule.rst
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+.. highlightlang:: c
+
+.. _capsules:
+
+Capsules
+--------
+
+.. index:: object: Capsule
+
+Refer to :ref:`using-capsules` for more information on using these objects.
+
+
+.. ctype:: PyCapsule
+
+ This subtype of :ctype:`PyObject` represents an opaque value, useful for C
+ extension modules who need to pass an opaque value (as a :ctype:`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.
+
+.. ctype:: PyCapsule_Destructor
+
+ The type of a destructor callback for a capsule. Defined as::
+
+ typedef void (*PyCapsule_Destructor)(PyObject *);
+
+ See :cfunc:`PyCapsule_New` for the semantics of PyCapsule_Destructor
+ callbacks.
+
+
+.. cfunction:: int PyCapsule_CheckExact(PyObject *p)
+
+ Return true if its argument is a :ctype:`PyCapsule`.
+
+
+.. cfunction:: PyObject* PyCapsule_New(void *pointer, const char *name, PyCapsule_Destructor destructor)
+
+ Create a :ctype:`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 :cfunc:`PyCapsule_Import`.
+
+
+.. cfunction:: void* PyCapsule_GetPointer(PyObject *capsule, const char *name)
+
+ 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 :cfunc:`strcmp` to compare capsule
+ names.
+
+
+.. cfunction:: PyCapsule_Destructor PyCapsule_GetDestructor(PyObject *capsule)
+
+ 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 :cfunc:`PyCapsule_IsValid` or
+ :cfunc:`PyErr_Occurred` to disambiguate.
+
+
+.. cfunction:: void* PyCapsule_GetContext(PyObject *capsule)
+
+ 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 :cfunc:`PyCapsule_IsValid` or
+ :cfunc:`PyErr_Occurred` to disambiguate.
+
+
+.. cfunction:: const char* PyCapsule_GetName(PyObject *capsule)
+
+ 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 :cfunc:`PyCapsule_IsValid` or
+ :cfunc:`PyErr_Occurred` to disambiguate.
+
+
+.. cfunction:: void* PyCapsule_Import(const char *name, int no_block)
+
+ 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. If *no_block* is true, import the module without blocking
+ (using :cfunc:`PyImport_ImportModuleNoBlock`). If *no_block* is false,
+ import the module conventionally (using :cfunc:`PyImport_ImportModule`).
+
+ Return the capsule's internal *pointer* on success. On failure, set an
+ exception and return *NULL*. However, if :cfunc:`PyCapsule_Import` failed to
+ import the module, and *no_block* was true, no exception is set.
+
+.. cfunction:: int PyCapsule_IsValid(PyObject *capsule, const char *name)
+
+ Determines whether or not *capsule* is a valid capsule. A valid capsule is
+ non-*NULL*, passes :cfunc:`PyCapsule_CheckExact`, has a non-*NULL* pointer
+ stored in it, and its internal name matches the *name* parameter. (See
+ :cfunc:`PyCapsule_GetPointer` for information on how capsule names are
+ compared.)
+
+ In other words, if :cfunc:`PyCapsule_IsValid` returns a true value, calls to
+ any of the accessors (any function starting with :cfunc:`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.
+
+.. cfunction:: int PyCapsule_SetContext(PyObject *capsule, void *context)
+
+ Set the context pointer inside *capsule* to *context*.
+
+ Return 0 on success. Return nonzero and set an exception on failure.
+
+.. cfunction:: int PyCapsule_SetDestructor(PyObject *capsule, PyCapsule_Destructor destructor)
+
+ Set the destructor inside *capsule* to *destructor*.
+
+ Return 0 on success. Return nonzero and set an exception on failure.
+
+.. cfunction:: int PyCapsule_SetName(PyObject *capsule, const char *name)
+
+ 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.
+
+.. cfunction:: int PyCapsule_SetPointer(PyObject *capsule, void *pointer)
+
+ 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.