Python __call__ special method practical example

Question

I know that __call__ method in a class is triggered when the instance of a class is called. However, I have no idea when I can use this special method, because

Answer 1

I find a good place to use callable objects, those that define __call__(), is when using the functional programming capabilities in Python, such as map(), filter(), reduce().

The best time to use a callable object over a plain function or a lambda function is when the logic is complex and needs to retain some state or uses other info that in not passed to the __call__() function.

Here's some code that filters file names based upon their filename extension using a callable object and filter().

Callable:

import os

class FileAcceptor(object):
    def __init__(self, accepted_extensions):
        self.accepted_extensions = accepted_extensions

    def __call__(self, filename):
        base, ext = os.path.splitext(filename)
        return ext in self.accepted_extensions

class ImageFileAcceptor(FileAcceptor):
    def __init__(self):
        image_extensions = ('.jpg', '.jpeg', '.gif', '.bmp')
        super(ImageFileAcceptor, self).__init__(image_extensions)

Usage:

filenames = [
    'me.jpg',
    'me.txt',
    'friend1.jpg',
    'friend2.bmp',
    'you.jpeg',
    'you.xml']

acceptor = ImageFileAcceptor()
image_filenames = filter(acceptor, filenames)
print image_filenames

Output:

['me.jpg', 'friend1.jpg', 'friend2.bmp', 'you.jpeg']