e-satis e-satis - 3 months ago 12
Python Question

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 ?

Answer

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