[Python-Dev] concerns regarding callable() method

[Alexey Borzenkov]

On 4/8/07, Guido van Rossum <guido at python.org> wrote:
> On 4/8/07, Paul Pogonyshev <pogonyshev at gmx.net> wrote:
> > Guido van Rossum wrote:
> > > What if someone passes a callable that doesn't have the expected signature?
> > Well, I don't know a way to catch such situations now, so removing
> > callable() will not make it worse (even if you don't know about hasattr
> > trick above.)
> My point is that it's futile to use callable() -- even if it passes,
> you have no assurance that you actually have a valid callback. So why
> bother with it at all? It's counter to the spirit of Python. If
> someone passes you a bad callback, they will see a traceback when you
> call it. Then they fix their program. That's how it's supposed to
> work.

But what if you need to do different things based on argument is
callable or not? Take for example "Dependency Injection" recipe:

    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/413268

It uses callable to differentiate whether it needs to use object as
singleton or to instantiate it on each request. I'm sure there might
be other uses for callable when it's really useful.