Objects/mappingproxyobject.c

Source:

cpython 3.14 @ ab2d84fe1023/Objects/mappingproxyobject.c ↗

mappingproxy is a read-only wrapper around a dict. It is the type of SomeClass.__dict__ and some_module.__dict__. The class types.MappingProxyType exposes it to Python code.

Map

Lines	Symbol	Role
1-60	mappingproxyobject struct	Holds a reference to the underlying mapping
61-180	mappingproxy_getitem, mappingproxy_keys, mappingproxy_len	Read-only dict protocol
181-260	mappingproxy_setitem	Raises TypeError: 'mappingproxy' object does not support item assignment
261-300	PyDictProxy_New	C API to wrap any mapping

Reading

Lookup

// CPython: Objects/mappingproxyobject.c:75 mappingproxy_getitem
static PyObject *
mappingproxy_getitem(mappingproxyobject *pp, PyObject *key)
{
    return PyObject_GetItem(pp->dict, key);
}

All read operations delegate to the underlying dict. There is no copy; the proxy and the dict share the same underlying hash table.

Write rejection

// CPython: Objects/mappingproxyobject.c:195 mappingproxy_setitem
static int
mappingproxy_setitem(mappingproxyobject *pp, PyObject *key, PyObject *value)
{
    PyErr_SetString(PyExc_TypeError,
                    "'mappingproxy' object does not support item assignment");
    return -1;
}

This is why SomeClass.__dict__['x'] = 1 raises TypeError; you must use setattr(SomeClass, 'x', 1) instead.

PyDictProxy_New

// CPython: Objects/mappingproxyobject.c:270 PyDictProxy_New
PyObject *
PyDictProxy_New(PyObject *mapping)
{
    mappingproxyobject *pp;
    pp = PyObject_New(mappingproxyobject, &PyDictProxy_Type);
    pp->dict = Py_NewRef(mapping);
    return (PyObject *)pp;
}

Called by type.__init__ to make cls.__dict__ read-only after the class is created.

gopy notes

mappingproxy is in objects/mapping_proxy.go. type.__dict__ returns a mappingproxy wrapping the type's internal attribute dict. This is needed for correct vars(SomeClass) behavior and for inspect.getmembers which iterates cls.__dict__. The underlying dict is a plain Go map exposed via the MappingProxy type.