[Python-3000] callable()

[Andrew Koenig]

> > That would be at odds with the approach taken with
> > list.__hash__ which has to be called in order to find-out it is not
> > hashable.

> That feature of __hash__ is just an unfortunate necessity.
> It arises because hashability is sometimes a "deep"
> property (i.e. it depends on the hashability of a
> container's contents as well as the nature of the
> container itself). No such thing applies to __iter__.

This example illustrates an important point:  Some object properties don't
correspond directly to the presence of a particular attribute, and can't
easily be made to do so.

In other words:

	Is it callable?  No problem, just check for __call__

	Is it iterable?  Well, you can't quite check for __iter__ because
	some iterable types don't have them.  Well, we can fix that problem:
	Change those types so that they have __iter__ to signal that they
	are iterable.

	Is it hashable?  That's a tough question to answer, because you
	have to inspect recursively all of the object's components.  So
	you can't just test for __hash__; you have to call it.

To my way of thinking, callable, iterable, and hashable are the same kind of
concept, and I wish Python would provide a uniform way of finding out
whether such concepts apply to an object.  That uniform way doesn't have to
be in __builtins__, but it would be nice for it to exist.