What is a "callable"?

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

From Python's sources object.c:

/* Test whether an object can be called */intPyCallable_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:

@Cacheddef 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 an object allows you to use round parenthesis ( ) and eventually pass some parameters, just like functions.

Every time you define a function python creates a callable object. In example, you could define the function func in these ways (it's the same):

class a(object):    def __call__(self, *args):        print 'Hello'func = a()# or ... def func(*args):    print 'Hello'

You could use this method instead of methods like doit or run, I think it's just more clear to see obj() than obj.doit()