Hi Greg, Hi List,
Greg wrote:
> class MyClass(B, Mixin):
> > "whatever"
> >
> > this leads to an MRO of MyClass -> B -> Mixin -> A -> object.
>
> If you do the tp_new stuff correctly at the C level, you can still
> create such a class. The only limitation is that if Mixin has a __new__
> method written in Python, it won't get called.
Yes, it is indeed possible to write a class like this. But that would be,
as mentioned in the comment, silly. Now there is nothing bad with being
silly once in a while, especially while programming that cannot be avoided
at times, I am just warning to use such a construct in a wider context.
Because the limitation is actually much more general: if the Mixin has any
method written in Python, it won't get called, unless you do a lot of heavy
lifting. Now why do you write something in C at all? Two reasons: once for
speed, which we can effectively exclude here because the super() construct
is slow, or because you want to interact with a library written in another
language. Unless your library knows about cooperative inheritance, which is
very unlikely, the code from class B is not aware that it is not supposed
to call the method inherited from A, but the one from the Mixin instead.
So by writing
class MyClass(B, Mixin):
"whatever"
I have major problems to understand what you actually want to achieve, what
you could not achieve by writing
class MyClass(Mixin, B):
"whatever"
Since one cannot achieve anything with the former that could not be
achieved by the latter, just with much less hassle, I would call the former
code silly.
That said, I might be wrong and there is indeed something one can achieve
with the former but not with the latter, I would be very pleased to hear
about this, so please come forward! (For sure, all is based on B and A
being written in C, not Python)
Cheers
Martin
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