Calling a Method on a Parent Class

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Classes and Objects Lab to test out this Shortcut in a live practice environment.

Problem

You want to invoke a method in a parent class in place of a method that has been overridden in a subclass.

Solution

To call a method in a parent (or superclass), use the super() function. For example:

class A(object):
    def spam(self):
        print('A.spam')

class B(A):
    def spam(self):
        print('B.spam')
        super().spam()      # Call parent spam()

A very common use of super() is in the handling of the __init__() method to make sure that parents are properly initialized:

class A(object):
    def __init__(self):
        self.x = 0

class B(A):
    def __init__(self):
        super().__init__()
        self.y = 1

Another common use of super() is in code that overrides any of Python’s special methods. For example:

class Proxy(object):
    def __init__(self, obj):
        self._obj = obj

    # Delegate attribute lookup to internal obj
    def __getattr__(self, name):
        return getattr(self._obj, name)

    # Delegate attribute assignment
    def __setattr__(self, name, value):
        if name.startswith('_'):
            super().__setattr__(name, value)    # Call original __setattr__
        else:
            setattr(self._obj, name, value)

In this code, the implementation of __setattr__() includes a name check. If the name starts with an underscore (_), it invokes the original implementation of __setattr__() using super(). Otherwise, it delegates to the internally held object self._obj ...