What is a "callable"?

From Python's sources object.c:

/* Test whether an object can be called */

int
PyCallable_Check(PyObject *x)
{
    if (x == NULL)
        return 0;
    if (PyInstance_Check(x)) {
        PyObject *call = PyObject_GetAttrString(x, "__call__");
        if (call == NULL) {
            PyErr_Clear();
            return 0;
        }
        /* Could test recursively but don't, for fear of endless
           recursion if some joker sets self.__call__ = self */
        Py_DECREF(call);
        return 1;
    }
    else {
        return x->ob_type->tp_call != NULL;
    }
}

It says:

1. If an object is an instance of some class then it is callable iff it has __call__ attribute.
2. Else the object x is callable iff x->ob_type->tp_call != NULL

Desciption of tp_call field:

ternaryfunc tp_call An optional pointer to a function that implements calling the object. This should be NULL if the object is not callable. The signature is the same as for PyObject_Call(). This field is inherited by subtypes.

You can always use built-in callable function to determine whether given object is callable or not; or better yet just call it and catch TypeError later. callable is removed in Python 3.0 and 3.1, use callable = lambda o: hasattr(o, '__call__') or isinstance(o, collections.Callable).

Example, a simplistic cache implementation:

class Cached:
    def __init__(self, function):
        self.function = function
        self.cache = {}

    def __call__(self, *args):
        try: return self.cache[args]
        except KeyError:
            ret = self.cache[args] = self.function(*args)
            return ret    

Usage:

@Cached
def ack(x, y):
    return ack(x-1, ack(x, y-1)) if x*y else (x + y + 1) 

Example from standard library, file site.py, definition of built-in exit() and quit() functions:

class Quitter(object):
    def __init__(self, name):
        self.name = name
    def __repr__(self):
        return 'Use %s() or %s to exit' % (self.name, eof)
    def __call__(self, code=None):
        # Shells like IDLE catch the SystemExit, but listen when their
        # stdin wrapper is closed.
        try:
            sys.stdin.close()
        except:
            pass
        raise SystemExit(code)
__builtin__.quit = Quitter('quit')
__builtin__.exit = Quitter('exit')

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