Python: How do I make a subclass from a superclass?

In Python, how do you make a subclass from a superclass?

Solution 1:

# Initialize using Parent
#
class MySubClass(MySuperClass):
    def __init__(self):
        MySuperClass.__init__(self)

Or, even better, the use of Python's built-in function, super() (see the Python 2/Python 3 documentation for it) may be a slightly better method of calling the parent for initialization:

# Better initialize using Parent (less redundant).
#
class MySubClassBetter(MySuperClass):
    def __init__(self):
        super(MySubClassBetter, self).__init__()

Or, same exact thing as just above, except using the zero argument form of super(), which only works inside a class definition:

class MySubClassBetter(MySuperClass):
    def __init__(self):
        super().__init__()

Solution 2:

A heroic little example:

class SuperHero(object): #superclass, inherits from default object
    def getName(self):
        raise NotImplementedError #you want to override this on the child classes

class SuperMan(SuperHero): #subclass, inherits from SuperHero
    def getName(self):
        return "Clark Kent"

class SuperManII(SuperHero): #another subclass
    def getName(self):
       return "Clark Kent, Jr."

if __name__ == "__main__":
    sm = SuperMan()
    print sm.getName()
    sm2 = SuperManII()
    print sm2.getName()

Solution 3:

class MySubClass(MySuperClass):
    def __init__(self):
        MySuperClass.__init__(self)

        # <the rest of your custom initialization code goes here>

The section on inheritance in the python documentation explains it in more detail

Solution 4:

class Class1(object):
    pass

class Class2(Class1):
    pass

Class2 is a sub-class of Class1

Solution 5:

In the answers above, the super is initialized without any (keyword) arguments. Often, however, you would like to do that, as well as pass on some 'custom' arguments of your own. Here is an example which illustrates this use case:

class SortedList(list):
    def __init__(self, *args, reverse=False, **kwargs):
        super().__init__(*args, **kwargs)       # Initialize the super class
        self.reverse = reverse
        self.sort(reverse=self.reverse)         # Do additional things with the custom keyword arguments

This is a subclass of list which, when initialized, immediately sorts itself in the direction specified by the reverse keyword argument, as the following tests illustrate:

import pytest

def test_1():
    assert SortedList([5, 2, 3]) == [2, 3, 5]

def test_2():
    SortedList([5, 2, 3], reverse=True) == [5, 3, 2]

def test_3():
    with pytest.raises(TypeError):
        sorted_list = SortedList([5, 2, 3], True)   # This doesn't work because 'reverse' must be passed as a keyword argument

if __name__ == "__main__":
    pytest.main([__file__])

Thanks to the passing on of *args to super, the list can be initialized and populated with items instead of only being empty. (Note that reverse is a keyword-only argument in accordance with PEP 3102).