CODEWITHSUNDEEP
pythonclasscall
Aaron Hall
Aaron Hall

Reputation: 395393

How do you call an instance of a class in Python?

This is inspired by a question I just saw, "Change what is returned by calling class instance", but was quickly answered with __repr__ (and accepted, so the questioner did not actually intend to call the instance).

Now calling an instance of a class can be done like this:

instance_of_object = object() 
instance_of_object()

but we'll get an error, something like TypeError: 'object' object is not callable.

This behavior is defined in the CPython source here.

So to ensure we have this question on Stackoverflow:

How do you actually call an instance of a class in Python?

Upvotes: 52

Views: 149722

Answers (3)

Prashant Marathay
Prashant Marathay

Reputation: 313

Up Votes for Everyone! Thanks for posting the question and thanks for answering.

I thought I would just share my implementation in case that helps others ...

I have a class (called RTS) and it contains an SQL Query that I access using a 'get'. The class works fine as an independent endpoint. Now I want to call that class from within the program.

Using the answer above I added the following:

class RTS(Resource):
    def __call__(self):
        print("In RTS")

    def get(self, user_id):
        try: ...

In order to call the class from elsewhere in the program I added:

getGR = RTS.get(self, user_unique_id)

Voila - I got the same info I could check on Postman returned within the program.

Upvotes: -1

aaronn
aaronn

Reputation: 478

The short answer is that the object class has no __call__ method (you can check that with "dir(object)"). When you create an instance of a class the __init__ method is called and when you call the instance, the __call__ method is called.

Upvotes: 1

Aaron Hall
Aaron Hall

Reputation: 395393

You call an instance of a class as in the following:

o = object() # create our instance
o() # call the instance

But this will typically give us an error. 

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'object' object is not callable

How can we call the instance as intended, and perhaps get something useful out of it?

We have to implement Python special method, __call__!

class Knight(object):
    def __call__(self, foo, bar, baz=None):
        print(foo)
        print(bar)
        print(bar)
        print(bar)
        print(baz)

Instantiate the class:

a_knight = Knight()

Now we can call the class instance:

a_knight('ni!', 'ichi', 'pitang-zoom-boing!')

which prints:

ni!
ichi
ichi
ichi
pitang-zoom-boing!

And we have now actually, and successfully, called an instance of the class!

Upvotes: 97

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