Inheritance

Classes can extend other classes. A subclass will have all the attributes and methods of a superclass. You can say the subclass inherits from the superclass.

A subclass can define new methods and attributes. It can also replace methods of the superclass.

The most important design principle when using inheritance is Liskovs Substitution Principle. It says that you should be able to use a subclass whereever the superclass is used.

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Example

Here you find an example using the Planet class defined earlier.

class UnstablePlanet(Planet):
    """A planet that explodes after some time"""

    def __init__(self, name: str, description: str, countdown: int):
        super().__init__(name, description)
        self.countdown = countdown

    def ticker(self):
        if self.countdown > 0:
            self.countdown -= 1
        else:
            print(f"BOOOM! {self.name} exploded")

The super() function returns an instance of the superclass, so that you can call the inherited methods, even if you are replacing them.

Using the subclass is very similar to using the superclass:

cornflake = UnstablePlanet('Cornflake', 'a crumbling planet', countdown=10)
while cornflake.coundown:
    cornflake.ticker()

Multiple Inheritance

Python allows multiple inheritance, often found under the term mixins. It is tricky, because the namespaces potentially collide. I advise to avoid multiple inheritance whenever possible.

A lifesaver is checking the method resolution order, see multiple_inheritance.py.

Warning

Caveat

Once you get the hang of object-oriented programming, inheritance is not that difficult. At some point, it is very tempting to define lots of subclasses, and class hierarchies with multiple levels. Most of the time, having one level of inheritance is enough. Many times you do not need inheritance at all, and you are better off using object composition.