Python/C API Reference Manual
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7.5.8 Weak Reference Objects

Python supports weak references as first-class objects. There are two specific object types which directly implement weak references. The first is a simple reference object, and the second acts as a proxy for the original object as much as it can.

int PyWeakref_Check(ob)

Return true if ob is either a reference or proxy object. New in version 2.2.

int PyWeakref_CheckRef(ob)

Return true if ob is a reference object. New in version 2.2.

int PyWeakref_CheckProxy(ob)

Return true if ob is a proxy object. New in version 2.2.

PyObject* PyWeakref_NewRef(PyObject *ob, PyObject *callback)

Return value: New reference.

Return a weak reference object for the object ob. This will always return a new reference, but is not guaranteed to create a new object; an existing reference object may be returned. The second parameter, callback, can be a callable object that receives notification when ob is garbage collected; it should accept a single paramter, which will be the weak reference object itself. callback may also be None or NULL. If ob is not a weakly-referencable object, or if callback is not callable, None, or NULL, this will return NULL and raise TypeError. New in version 2.2.

PyObject* PyWeakref_NewProxy(PyObject *ob, PyObject *callback)

Return value: New reference.

Return a weak reference proxy object for the object ob. This will always return a new reference, but is not guaranteed to create a new object; an existing proxy object may be returned. The second parameter, callback, can be a callable object that receives notification when ob is garbage collected; it should accept a single paramter, which will be the weak reference object itself. callback may also be None or NULL. If ob is not a weakly-referencable object, or if callback is not callable, None, or NULL, this will return NULL and raise TypeError. New in version 2.2.

PyObject* PyWeakref_GetObject(PyObject *ref)

Return value: Borrowed reference.

Returns the referenced object from a weak reference, ref. If the referent is no longer live, returns NULL. New in version 2.2.

PyObject* PyWeakref_GET_OBJECT(PyObject *ref)

Return value: Borrowed reference.

Similar to PyWeakref_GetObject(), but implemented as a macro that does no error checking. New in version 2.2.