[Python-3000] Could isinstance/issubclass overriding be dangerous?

[Greg Ewing]

Guido van Rossum wrote:

> Unless they were looking at classic classes, why wouldn't they be
> using the specific Py<Type>_Check() macros?

I'm thinking of Pyrex code. One of the goals of Pyrex
is that you should be able to write it without needing
to know about the Python/C API. One of the places that's
not possible at the moment is in binary operator methods,
where you need to write things like

   cdef class C:

     def __add(x, y):
       if PyObject_TypeCheck(x, C):
         # we're the left operand
       elif PyObject_TypeCheck(y, C):
         # we're the right operand

I'd like to be able to provide optimised access to
isinstance() so that you can think in Python instead of
C and write

     def __add(x, y):
       if isinstance(x, C):
         # we're the left operand
       elif isinstance(y, C):
         # we're the right operand

But it seems like isinstance() is already the wrong thing
to use for this, and there is currently *no* Python-level
function that does what is needed here.

So can we please have another couple of functions that just
do a simple, reliable concrete type test?

--
Greg