What is a “callable” in Python?

/* 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;
    }
}

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

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

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')

class Foo:
  def __call__(self):
    print 'called'

foo_instance = Foo()
foo_instance() #this is calling the __call__ method

>>> type(int).__call__(int)
0
>>> type(object).__call__(object)
<object object at 0x523460>

class a(object):
    def __call__(self, *args):
        print 'Hello'

func = a()

# or ...
def func(*args):
    print 'Hello'

class Adder(object):
  def __init__(self, val):
    self.val = val

  def __call__(self, val):
    return self.val + val

func = Adder(5)
print func(3)

[nigel@k9 ~]$ python
Python 2.5 (r25:51908, Nov  6 2007, 15:55:44)
[GCC 4.1.2 20070925 (Red Hat 4.1.2-27)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 'aaa'()    # <== Here we attempt to call a string.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object is not callable
>>>

>>> class Foo:
...  pass
...
>>> class Bar(object):
...  pass
...
>>> type(Foo).__call__(Foo)
<__main__.Foo instance at 0x711440>
>>> type(Bar).__call__(Bar)
<__main__.Bar object at 0x712110>
>>> def foo(bar):
...  return bar
...
>>> type(foo).__call__(foo, 42)
42

>>> class Foo(object):
...  def __call__(self):
...   return 42
...
>>> f = Foo()
>>> f()
42