Is types.InstanceType no longer valid with Python 2.2

Alex Martelli aleax at
Fri Jan 4 14:29:28 EST 2002

Skip Montanaro wrote:
> How can foo be a classic class with no bases, f1 be both an instance of
> foo
> and of object, but foo not be a subclass of object?  Just to pull all the

Implementatively speaking, it boils down to half a line in source file:

    387 int
    388 PyType_IsSubtype(PyTypeObject *a, PyTypeObject *b)
    389 {
    392         if (!(a->tp_flags & Py_TPFLAGS_HAVE_CLASS))
    393                 return b == a || b == &PyBaseObject_Type;

The "half a line" being the part of line 393 from the || to the ; -- 
basically, this says "if the second argument is builtin `object', then, 
yes, type a IS a subtype of it, WHATEVER type a may be".

The insinstance built-in boils down to PyType_IsSubtype when the second 
argument is not a class -- in Objects/abstract.c:

   1890 int
   1891 PyObject_IsInstance(PyObject *inst, PyObject *cls)
   1892 {
   1897         if (PyClass_Check(cls) && PyInstance_Check(inst)) {
   1902         else if (PyType_Check(cls)) {
   1903                 retval = PyObject_TypeCheck(inst, (PyTypeObject 

OTOH,  PyObject_IsSubclass (lines 1946 ff of the same source file) does
not call PyType_IsSubtype under any circumstances -- rather, it works
via abstract_get_bases (lines 1842 ff, same source).

I'm not implying this is RIGHT, mind you -- I have no opinion on that yet; 
it's just what  IS happening in this case you're observing.


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