<s>[INST] What is a "callable" in Python? 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 ?
 [/INST] 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

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