Reference Counting

The functions and macros in this section are used for managing reference counts of Python objects.

Py_ssize_t Py_REFCNT(PyObject *o)

Get the reference count of the Python object o.

Use the Py_SET_REFCNT() function to set an object reference count.

Changed in version 3.11: The parameter type is no longer const PyObject*.

Changed in version 3.10: Py_REFCNT() is changed to the inline static function.

void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt)

Set the object o reference counter to refcnt.

New in version 3.9.

void Py_INCREF(PyObject *o)

Increment the reference count for object o.

This function is usually used to convert a borrowed reference to a strong reference in-place. The Py_NewRef() function can be used to create a new strong reference.

The object must not be NULL; if you aren’t sure that it isn’t NULL, use Py_XINCREF().

void Py_XINCREF(PyObject *o)

Increment the reference count for object o. The object may be NULL, in which case the macro has no effect.

See also Py_XNewRef().

PyObject *Py_NewRef(PyObject *o)

Part of the Stable ABI since version 3.10.

Create a new strong reference to an object: increment the reference count of the object o and return the object o.

When the strong reference is no longer needed, Py_DECREF() should be called on it to decrement the object reference count.

The object o must not be NULL; use Py_XNewRef() if o can be NULL.

For example:

Py_INCREF(obj);
self->attr = obj;

can be written as:

self->attr = Py_NewRef(obj);

See also Py_INCREF().

New in version 3.10.

PyObject *Py_XNewRef(PyObject *o)

Part of the Stable ABI since version 3.10.

Similar to Py_NewRef(), but the object o can be NULL.

If the object o is NULL, the function just returns NULL.

New in version 3.10.

void Py_DECREF(PyObject *o)

Decrement the reference count for object o.

If the reference count reaches zero, the object’s type’s deallocation function (which must not be NULL) is invoked.

This function is usually used to delete a strong reference before exiting its scope.

The object must not be NULL; if you aren’t sure that it isn’t NULL, use Py_XDECREF().

Warning

The deallocation function can cause arbitrary Python code to be invoked (e.g. when a class instance with a __del__() method is deallocated). While exceptions in such code are not propagated, the executed code has free access to all Python global variables. This means that any object that is reachable from a global variable should be in a consistent state before Py_DECREF() is invoked. For example, code to delete an object from a list should copy a reference to the deleted object in a temporary variable, update the list data structure, and then call Py_DECREF() for the temporary variable.

void Py_XDECREF(PyObject *o)

Decrement the reference count for object o. The object may be NULL, in which case the macro has no effect; otherwise the effect is the same as for Py_DECREF(), and the same warning applies.

void Py_CLEAR(PyObject *o)

Decrement the reference count for object o. The object may be NULL, in which case the macro has no effect; otherwise the effect is the same as for Py_DECREF(), except that the argument is also set to NULL. The warning for Py_DECREF() does not apply with respect to the object passed because the macro carefully uses a temporary variable and sets the argument to NULL before decrementing its reference count.

It is a good idea to use this macro whenever decrementing the reference count of an object that might be traversed during garbage collection.

Changed in version 3.12: The macro argument is now only evaluated once. If the argument has side effects, these are no longer duplicated.

void Py_IncRef(PyObject *o)

Part of the Stable ABI.

Increment the reference count for object o. A function version of Py_XINCREF(). It can be used for runtime dynamic embedding of Python.

void Py_DecRef(PyObject *o)

Part of the Stable ABI.

Decrement the reference count for object o. A function version of Py_XDECREF(). It can be used for runtime dynamic embedding of Python.

Py_SETREF(dst, src)

Macro safely decrementing the dst reference count and setting dst to src.

As in case of Py_CLEAR(), “the obvious” code can be deadly:

Py_DECREF(dst);
dst = src;

The safe way is:

Py_SETREF(dst, src);

That arranges to set dst to src _before_ decrementing reference count of dst old value, so that any code triggered as a side-effect of dst getting torn down no longer believes dst points to a valid object.

New in version 3.6.

Changed in version 3.12: The macro arguments are now only evaluated once. If an argument has side effects, these are no longer duplicated.

Py_XSETREF(dst, src)

Variant of Py_SETREF macro that uses Py_XDECREF() instead of Py_DECREF().

New in version 3.6.

Changed in version 3.12: The macro arguments are now only evaluated once. If an argument has side effects, these are no longer duplicated.