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

Antoine Pitrou solipsis at pitrou.net
Thu Sep 23 20:01:33 CEST 2010

Le jeudi 23 septembre 2010 à 19:51 +0200, Tarek Ziadé a écrit :
> On Thu, Sep 23, 2010 at 6:32 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> ...
> > This feature already exists, as you mention, using issubclass() or
> > isinstance(). What you are asking for is a different feature: check that
> > a class has an appropriate implementation of the advertised
> > capabilities. Traditionally, this is best left to unit testing (or other
> > forms of test-based checking).
> >
> > Do you have an use case where unit testing would not be appropriate for
> > this?
> Why are you thinking about unit tests  ? Don't you ever use
> issubclass/isinstance in your programs ?

Sorry, you don't seem to be answering the question.
Why wouldn't the implementor of the class use unit tests to check that
his/her class implements the desired ABC?

> Checking signatures using ABC when you create a plugin system is one
> use case for instance.

Again, why do you want to check signatures? Do you not trust plugin
authors to write plugins?

Also, why do you think checking signatures is actually useful? It only
checks that the signature is right, not that the expected semantics are
observed. The argument for checking method signature in advance is as
weak as the argument for checking types at compile time.

> > It depends on the arguments. And the implementation could definitely use
> > *args or **kwargs arguments, especially if it acts as a proxy.
> Sure but ISTM that most of the time signatures are well defined, and
> proxies lives in an upper layer.

Not really. If I write a file object wrapper that proxies some methods
to an other file object, I don't want to re-type all method signatures
(including default args) by hand.



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