What is a “callable”?

Question

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

Answer 1

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

Answer 2

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

Answer 3

__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)

Answer 4

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
>>> 

Answer 5

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.

Answer 6

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