getattr and __mro__ (was Re: [Python-Dev] PEP 246, redux)

[Thomas Heller]

Armin Rigo <arigo at tunes.org> writes:

> ... is that the __adapt__() and __conform__() methods should work just
> like all other special methods for new-style classes.  The confusion
> comes from the fact that the reference implementation doesn't do that.
> It should be fixed by replacing:
>
>    conform = getattr(type(obj), '__conform__', None)
>
> with:
>
>    for basecls in type(obj).__mro__:
>        if '__conform__' in basecls.__dict__:
>            conform = basecls.__dict__['__conform__']
>            break
>    else:
>        # not found
>
> and the same for '__adapt__'.
>
> The point about tp_xxx slots is that when implemented in C with slots, you get
> the latter (correct) effect for free.  This is how metaconfusion is avoided in
> post-2.2 Python.  Using getattr() for that is essentially broken.  Trying to
> call the method and catching TypeErrors seems pretty fragile -- e.g. if you
> are calling a __conform__() which is implemented in C you won't get a Python
> frame in the traceback either.

I'm confused.  Do you mean that

   getattr(obj, "somemethod")(...)

does something different than

   obj.somemethod(...)

with new style class instances?  Doesn't getattr search the __dict__'s
along the __mro__ list?

Thomas