CODEWITHSUNDEEP
pythonoopinheritance
Brendan
Brendan

Reputation: 2391

Why does Python 2's `super` require explicit class and self arguments?

This Python 2 example:

class LoggingDict(dict):
    # Simple example of extending a builtin class
    def __setitem__(self, key, value):
        logging.info('Setting %r to %r' % (key, value))
        super(LoggingDict, self).__setitem__(key, value)

and this Python 3 example:

class LoggingDict(dict):
    # Simple example of extending a builtin class
    def __setitem__(self, key, value):
        logging.info('Setting %r to %r' % (key, value))
        super().__setitem__(key, value)

illustrate the fact that Python 2's super requires explicit class and self arguments (but Python 3's doesnt). Why is that? It seems like an irritating limitation.

Upvotes: 0

Views: 146

Answers (1)

Brendan
Brendan

Reputation: 2391

The link in AKS' comment provides the answer here:

Lets say in the Python 2 example I thought "I don't like that explicit class reference. What if I change the name of the class or move this code and forget to update it?". Lets say I thought, a-ha, I'll replace the explicit class name with self.__class__ and wrote:

class LoggingDict(dict):
    # Simple example of extending a builtin class
    def __setitem__(self, key, value):
        logging.info('Setting %r to %r' % (key, value))
        super(self.__class__, self).__setitem__(key, value)

Now I create a subclass called SpecialisedLoggingDict of LoggingDict (which doesn't override __setitem__), instantiate it and call __setitem__ on it.

Now self refers to an instance of SpecialisedLoggingDict, so the super returns LoggingDict, and we go straight back into LoggingDict.__setitem__, entering infinite recursion.

The essential point is that in Python 2 a method doesn't really know which class it was defined in, it only knows the class of the instance on which it's being called. Python 3 does compile-time "magic", adding a __class__ cell to functions so that super() can be used without the explicit class reference.

Upvotes: 1

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