Cell Objects
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"Cell" objects are used to implement variables referenced by multiple
scopes. For each such variable, a cell object is created to store the
value; the local variables of each stack frame that references the
value contain a reference to the cells from outer scopes which also
use that variable.  When the value is accessed, the value contained in
the cell is used instead of the cell object itself.  This de-
referencing of the cell object requires support from the generated
byte-code; these are not automatically de-referenced when accessed.
Cell objects are not likely to be useful elsewhere.

type PyCellObject

   The C structure used for cell objects.

PyTypeObject PyCell_Type

   The type object corresponding to cell objects.

int PyCell_Check(PyObject *ob)

   Return true if *ob* is a cell object; *ob* must not be "NULL".
   This function always succeeds.

PyObject *PyCell_New(PyObject *ob)
    *Return value: New reference.*

   Create and return a new cell object containing the value *ob*. The
   parameter may be "NULL".

PyObject *PyCell_Get(PyObject *cell)
    *Return value: New reference.*

   Return the contents of the cell *cell*, which can be "NULL". If
   *cell* is not a cell object, returns "NULL" with an exception set.

PyObject *PyCell_GET(PyObject *cell)
    *Return value: Borrowed reference.*

   Return the contents of the cell *cell*, but without checking that
   *cell* is non-"NULL" and a cell object.

int PyCell_Set(PyObject *cell, PyObject *value)

   Set the contents of the cell object *cell* to *value*.  This
   releases the reference to any current content of the cell. *value*
   may be "NULL".  *cell* must be non-"NULL".

   On success, return "0". If *cell* is not a cell object, set an
   exception and return "-1".

void PyCell_SET(PyObject *cell, PyObject *value)

   Sets the value of the cell object *cell* to *value*.  No reference
   counts are adjusted, and no checks are made for safety; *cell* must
   be non-"NULL" and must be a cell object.
