6: Object Oriented Programming Python

Dunder methods

Methods that have the double underscore on each side __ are dunder methods.

Dunder methods help us to write the implementation of Python specific functions on objects created by our class.

class Toy():
    def __init__(self, color, age):
        self.color = color
        self.age = age

action_figure = Toy('red', 3)
print(action_figure.__str__()) # <__main__.Toy object at 0x7f8b8b8b8d50>
print(str(action_figure)) # <__main__.Toy object at 0x7f8b8b8b8d50> -- the same as before
copy

In the above we can see that the built-in function str is the implementation of __str__ method.

The documentation has a list of dunder methods.

In general, there are a specific few cases where you will use them.

To define your own implementation:

class Toy():
    def __init__(self, color, age):
        self.color = color
        self.age = age

    def __str__(self):
        return f'{self.color} toy'

action_figure = Toy('red', 3)
print(action_figure.__str__()) # red toy
print(str(action_figure)) # red toy -- the same as before
copy

Be careful of how you implement these things. You may want to make adjustments for particular comparison operators or methods for lists, dicts.

Some interesting methods:

Multiple Inheritance

# This is now inheriting from both Wizard and Archer
class Hybrid(Wizard, Archer):
    pass
copy

Module Resolution Order

class A:
    num = 10

class B(A):
    pass

class C(A):
    num = 1

class D(B, C):
    pass
copy

MRO is a rule for which how Python will resolve the inheritance hierarchy.

If we run print(D.num) from the above, we get one.

You can check D.mro() to see the order.

You want to be conscious of MRO. MRO uses depth first search to resolve the order.