Are functions objects in Python?

Question

I always hear this statement in Python (for topics such as decorators, etc. when you are passing functions, etc.) but have never really seen an elaboration on this.

Answer 1

You are looking for the __call__ method. Function objects have that method:

>>> def foo(): pass
... 
>>> foo.__call__

Not that the Python interpreter loop actually makes use of that method when encountering a Python function object; optimisations in the implementation jump straight to the contained bytecode in most cases.

But you can use that on your own custom class:

class Callable(object):
    def __init__(self, name):
        self.name = name

    def __call__(self, greeting):
        return '{}, {}!'.format(greeting, self.name)

Demo:

>>> class Callable(object):
...     def __init__(self, name):
...         self.name = name
...     def __call__(self, greeting):
...         return '{}, {}!'.format(greeting, self.name)
... 
>>> Callable('World')('Hello')
'Hello, World!'

Python creates function objects for you when you use a def statement, or you use a lambda expression:

>>> def foo(): pass
... 
>>> foo

>>> lambda: None
 at 0x106d90668>

You can compare this to creating a string or an integer or a list using literal syntax:

listobject = [1, 'two']

The above creates 3 objects without ever calling a type, Python did that all for you based on the syntax used. The same applies to functions.

Creating one yourself can be a little more complex; you need to have a code object and reference to a global namespace, at the very least:

>>> function_type = type(lambda: None)
>>> function_type

>>> function_type(foo.__code__, globals(), 'bar')

Here I created a function object by reusing the function type, taking the code object from the foo function; the function type is not a built-in name but the type really does exist and can be obtained by calling type() on an existing function instance.

I also passed in the global namespace of my interpreter, and a name; the latter is an optional argument; the name is otherwise taken from the code object.