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)

Part of the Stable ABI since version 3.14.

Get the reference count of the Python object o.

Note that the returned value may not actually reflect how many references to the object are actually held. For example, some objects are immortal and have a very high refcount that does not reflect the actual number of references. Consequently, do not rely on the returned value to be accurate, other than a value of 0 or 1.

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

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

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

void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt)

Set the object o reference counter to refcnt.

On Python build with Free Threading, if refcnt is larger than UINT32_MAX, the object is made immortal.

This function has no effect on immortal objects.

Added in version 3.9.

Changed in version 3.12: Immortal objects are not modified.

void Py_INCREF(PyObject *o)

Indicate taking a new strong reference to object o, indicating it is in use and should not be destroyed.

This function has no effect on immortal objects.

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.

When done using the object, release is by calling Py_DECREF().

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

Do not expect this function to actually modify o in any way. For at least some objects, this function has no effect.

Changed in version 3.12: Immortal objects are not modified.

void Py_XINCREF(PyObject *o)

Similar to Py_INCREF(), but the object o can be NULL, in which case this 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: call Py_INCREF() on o and return the object o.

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

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().

Added 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.

Added in version 3.10.

void Py_DECREF(PyObject *o)

Release a strong reference to object o, indicating the reference is no longer used.

This function has no effect on immortal objects.

Once the last strong reference is released (i.e. the object’s reference count reaches 0), 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().

Do not expect this function to actually modify o in any way. For at least some objects, this function has no effect.

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.

Changed in version 3.12: Immortal objects are not modified.

void Py_XDECREF(PyObject *o)

Similar to Py_DECREF(), but the object o can be NULL, in which case this has no effect. The same warning from Py_DECREF() applies here as well.

void Py_CLEAR(PyObject *o)

Release a strong reference 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 releasing the reference.

It is a good idea to use this macro whenever releasing a reference to 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.

Indicate taking a new strong reference to 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.

Release a strong reference to object o. A function version of Py_XDECREF(). It can be used for runtime dynamic embedding of Python.

Py_SETREF(dst, src)

Macro safely releasing a strong reference to object dst 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_ releasing the reference to the old value of dst, so that any code triggered as a side-effect of dst getting torn down no longer believes dst points to a valid object.

Added 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().

Added 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.