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 a mistake with parenthesis, resulting in an ‘object is not callable’ exception. What’s more, using __init__ and __new__ lead to wonder what this bloody __call__ can be used for.

Could you give me some explanations, including examples with the magic method ?

Answer 1

A callable is anything that can be called.

The built-in callable (PyCallable_Check in objects.c) checks if the argument is either:

• an instance of a class with a __call__ method or
• is of a type that has a non null tp_call (c struct) member which indicates callability otherwise (such as in functions, methods etc.)

The method named __call__ is (according to the documentation)

Called when the instance is ”called” as a function

Example

class Foo:   def __call__(self):     print 'called'  foo_instance = Foo() foo_instance() #this is calling the __call__ method