Reference Counting — Python 3.13.2 documentation
Description: The functions and macros in this section are used for managing reference counts of Python objects.

# 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.

  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.