Question

I want to override a method of the class dict in Python, a simple one: update. Let’s say I want to create a MyDict class which is identical to standard dict except that it can be updated with dicts that have to contain at least 10 elements.

So I would proceed like:

def update(self, newdict):
    if len(newdict) <= 10: raise Exception
    self.update(newdict)

But in the inner call to update, obviously Python calls the overridden function and not the original one. Is there a way to avoid this situations other than simply changing the function names?

Answer 1

You need to call update on the superclass, supplying an instance of the subclass as self.

def update(self, newdict):
    if len(newdict) <= 10: raise Exception
    dict.update(self, newdict)

You can also use super() to determine the superclass at runtime:

def update(self, newdict):
    if len(newdict) <= 10: raise Exception
    super(MyDict, self).update(newdict)

In Python 3, you can omit the parameters to super():

def update(self, newdict):
    if len(newdict) <= 10: raise Exception
    super().update(newdict)