What is a “callable”?

Question

Now that it\'s clear what a metaclass is, there is an associated concept that I use all the time without knowing what it really means.

I suppose everybody made once

Answer 1

Let me explain backwards:

Consider this...

foo()

... as syntactic sugar for:

foo.__call__()

Where foo can be any object that responds to __call__. When I say any object, I mean it: built-in types, your own classes and their instances.

In the case of built-in types, when you write:

int('10')
unicode(10)

You're essentially doing:

int.__call__('10')
unicode.__call__(10)

That's also why you don't have foo = new int in Python: you just make the class object return an instance of it on __call__. The way Python solves this is very elegant in my opinion.