Objects/cellobject.c (part 3)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/cellobject.c ↗

This annotation covers the cell object implementation. See python_ceval60_detail for MAKE_CELL, LOAD_DEREF, and STORE_DEREF.

Map

Lines	Symbol	Role
1-60	PyCellObject layout	Single pointer ob_ref wrapping a value
61-120	PyCell_New	Create a cell with optional initial content
121-180	PyCell_Get / PyCell_Set	Thread-safe read/write
181-300	cell.__repr__ / __eq__	String representation and equality

Reading

PyCellObject layout

// CPython: Objects/cellobject.c:10 PyCellObject
typedef struct {
    PyObject_HEAD
    PyObject *ob_ref;  /* NULL if cell is empty */
} PyCellObject;

A cell is just a mutable box containing one reference. ob_ref == NULL means the variable is unbound (not yet assigned, or del-ed). LOAD_DEREF raises UnboundLocalError when ob_ref is NULL.

PyCell_New

// CPython: Objects/cellobject.c:20 PyCell_New
PyObject *
PyCell_New(PyObject *obj)
{
    PyCellObject *op = (PyCellObject *)PyObject_GC_New(PyCellObject, &PyCell_Type);
    op->ob_ref = Py_XNewRef(obj);
    _PyObject_GC_TRACK(op);
    return (PyObject *)op;
}

Cell objects are GC-tracked because they can participate in reference cycles (a closure that references itself). Py_XNewRef handles obj == NULL (empty cell).

Thread safety

// CPython: Objects/cellobject.c:60 PyCell_Set
int
PyCell_Set(PyObject *op, PyObject *value)
{
    PyObject *old;
    if (!PyCell_Check(op)) {
        PyErr_BadInternalCall();
        return -1;
    }
    old = PyCell_GET(op);
    PyCell_SET(op, Py_XNewRef(value));
    Py_XDECREF(old);
    return 0;
}

PyCell_SET is a direct macro: ((PyCellObject*)op)->ob_ref = v. The decref of the old value happens after the new value is stored, avoiding a window where the value is unreachable. Under the GIL, this is safe; with per-object locks in 3.13+, cell access requires holding the cell's lock.

gopy notes

PyCellObject is objects.Cell in objects/cell.go. ob_ref is a *objects.Object. PyCell_Set updates the pointer. LOAD_DEREF reads cell.Ref and raises objects.UnboundLocalError if nil. STORE_DEREF sets cell.Ref.