In the file dictobject.c
, you see in line 1795ff. the relevant code:
static int
dict_update_common(PyObject *self, PyObject *args, PyObject *kwds, char *methname)
{
PyObject *arg = NULL;
int result = 0;
if (!PyArg_UnpackTuple(args, methname, 0, 1, &arg))
result = -1;
else if (arg != NULL) {
_Py_IDENTIFIER(keys);
if (_PyObject_HasAttrId(arg, &PyId_keys))
result = PyDict_Merge(self, arg, 1);
else
result = PyDict_MergeFromSeq2(self, arg, 1);
}
if (result == 0 && kwds != NULL) {
if (PyArg_ValidateKeywordArguments(kwds))
result = PyDict_Merge(self, kwds, 1);
else
result = -1;
}
return result;
}
This tells us that if the object has an attribute keys
, the code which is called is a mere merge. The code called there (l. 1915 ff.) makes a distinction between real dicts and other objects. In the case of real dicts, the items are read out with PyDict_GetItem()
, which is the "most inner interface" to the object and doesn't bother using any user-defined methods.
So instead of inheriting from dict
, you should use the UserDict
module.