Python 3.6.5 Documentation >  Reference Counting

Reference Counting
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The macros in this section are used for managing reference counts of
Python objects.

void Py_INCREF(PyObject *o)

Increment the reference count for object *o*. 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.

void Py_DECREF(PyObject *o)

Decrement the reference count for object *o*. The object must not
be *NULL*; if you aren’t sure that it isn’t *NULL*, use
"Py_XDECREF()". If the reference count reaches zero, the object’s
type’s deallocation function (which must not be *NULL*) is invoked.

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 value
of a variable that might be traversed during garbage collection.

The following functions are for runtime dynamic embedding of Python:
"Py_IncRef(PyObject *o)", "Py_DecRef(PyObject *o)". They are simply
exported function versions of "Py_XINCREF()" and "Py_XDECREF()",
respectively.

The following functions or macros are only for use within the
interpreter core: "_Py_Dealloc()", "_Py_ForgetReference()",
"_Py_NewReference()", as well as the global variable "_Py_RefTotal".