[python] What is a "callable"?

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 ?

Callable is a type or class of "Build-in function or Method" with a method call

>>> type(callable)
<class 'builtin_function_or_method'>
>>>

Example: print is a callable object. With a build-in function __call__ When you invoke the print function, Python creates an object of type print and invokes its method __call__ passing the parameters if any.

>>> type(print)
<class 'builtin_function_or_method'>
>>> print.__call__(10)
10
>>> print(10)
10
>>>

Thank you. Regards, Maris

Quite simply, a "callable" is something that can be called like a method. The built in function "callable()" will tell you whether something appears to be callable, as will checking for a call property. Functions are callable as are classes, class instances can be callable. See more about this here and here.

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 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()

To check function or method of class is callable or not that means we can call that function.

Class A:
    def __init__(self,val):
        self.val = val
    def bar(self):
        print "bar"

obj = A()      
callable(obj.bar)
True
callable(obj.__init___)
False
def foo(): return "s"
callable(foo)
True
callable(foo())
False

A Callable is an object that has the __call__ method. This means you can fake callable functions or do neat things like Partial Function Application where you take a function and add something that enhances it or fills in some of the parameters, returning something that can be called in turn (known as Currying in functional programming circles).

Certain typographic errors will have the interpreter attempting to call something you did not intend, such as (for example) a string. This can produce errors where the interpreter attempts to execute a non-callable application. You can see this happening in a python interpreter by doing something like the transcript below.

[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
>>> 

__call__ makes any object be callable as a function.

This example will output 8:

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

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

func = Adder(5)
print func(3)

In Python a callable is an object which type has a __call__ method:

>>> 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

As simple as that :)

This of course can be overloaded:

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

callables implement the __call__ special method so any object with such a method is callable.

Let me explain backwards:

Consider this...

foo()

... as syntactic sugar for:

foo.__call__()

Where foo can be any object that responds to __call__. When I say any object, I mean it: built-in types, your own classes and their instances.

In the case of built-in types, when you write:

int('10')
unicode(10)

You're essentially doing:

int.__call__('10')
unicode.__call__(10)

That's also why you don't have foo = new int in Python: you just make the class object return an instance of it on __call__. The way Python solves this is very elegant in my opinion.

It's something you can put "(args)" after and expect it to work. A callable is usually a method or a class. Methods get called, classes get instantiated.