Question

In python you can make instances callable by implementing the __call__ method. For example

class Blah:
    def __call__(self):
        print "hello"

obj = Blah()
obj()

But I can also implement a method of my own, say ‘run’:

class Blah:
    def run(self):
        print "hello"

obj = Blah()
obj.run()

When should I implement __call__?

Answer 1

This is hard to answer. My opinion is that you should never define __call__ unless your actual goal is to create a function. It’s not something you would do after you’ve already created a traditional object.

In other words, if you’re starting out thinking “I’m going to create an object” you should never end up implementing __call__. If, on the other hand, you start out thinking “I’m going to create a function… but I wish I could use the object framework to let my function have some state” then you could create the function as an object with state, and define __call__ to make it act like a function.

I want to include one final bit of advice which was provided by @Paul Manta in a comment to the question, where he wrote:

If you’re unsure if you need __call__, then you don’t need __call__

I think that’s sound advice.