[Python-ideas] ABC: what about the method arguments ?

[Guido van Rossum]

That is not a new idea. So far I have always rejected it because I
worry about both false positives and false negatives. Trying to
enforce that the method *behaves* as it should (or even its return
type) is hopeless; there can be a variety of reasons to modify the
argument list while still conforming to (the intent of) the interface.
I also worry that it will slow everything down.

That said, if you want to provide a standard mechanism that can
*optionally* be turned on to check argument conformance, e.g. by using
a class or method decorator on the subclass, I would be fine with that
(as long as it runs purely at class-definition time; it shouldn't slow
down class instantiation or method calls). It will probably even find
some bugs. It will also surely have to be tuned to avoid certain
classes false positives.

--Guido

On Thu, Sep 23, 2010 at 7:37 AM, Tarek Ziadé <ziade.tarek at gmail.com> wrote:
> Hello,
>
> ABC __subclasshook__ implementations will only check that the method
> is present in the class. That's the case for example in
> collections.Container. It will check that the __contains__ method is
> present but that's it. It won't check that the method has only one
> argument. e.g. __contains__(self, x)
>
> The problem is that the implemented method could have a different list
> of arguments and will eventually fail.
>
> Using inspect, we could check in __subclasshook__ that the arguments
> defined are the same than the ones defined in the abstractmethod.--
> the name and the ordering.
>
> I can even think of a small function in ABC for that:
> same_signature(method1, method2) => True or False
>
> Regards
> Tarek
>
> --
> Tarek Ziadé | http://ziade.org
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-- 
--Guido van Rossum (python.org/~guido)