SOLID Python part 4: Liskov Substitution Principle

Liskov Substitution Principle with Python

Derived classes must be substitutable for their base classes.

The Liskov Substitution Principle (LSP) was created by Barbara Liskov in 1987:

What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T. [@Liskov1987]

This is a whole mouth full, so I will try to explain it with an example.

Liskov Substitution Principle example

In this example we’re going to let everyone in Middle Earth dance. First we create an abstract base class called MiddleEarthInhabitant, after which we create an Human and a Hobbit

from abc import ABC

class MiddleEarthInhabitant(ABC):
    def dance(self):
        ...

class Human(MiddleEarthInhabitant):
    def dance(self):
        print("Going wild on the dance floor.")

class Hobbit(MiddleEarthInhabitant):
    def dance(self):
        print("Look at those big feet go.")

class Party:
    def __init__(self, guests: List[MiddleEarthInhabitant]):
        self._guests = guests

    def que_music(self):
        for guest in self._guests:
            guest.dance()

Now when we want to create an Orc, which is also a Middle Earth inhabitant, we hit a problem: Orcs are fighters, not dancers. This means the LSP would be violated.

In order to fix this we have to create a separate interface (Which is an abstract base class in Python) and use that to create the party people.

from abc import ABC

class MiddleEarthInhabitant(ABC):
    ...

class Dancer(ABC):
    def dance(self):
        ...

class Human(MiddleEarthInhabitant, Dancer):
    def dance(self):
        print("Going wild on the dance floor.")


class Hobbit(MiddleEarthInhabitant, Dancer):
    def dance(self):
        print("Look at those big feet go.")

class Party:
    def __init__(self, guests: List[Dancer]):
        self._guests = guests

    def que_music(self):
        for guest in self._guests:
            guest.dance()

Now we can create an Orc, which still is a MiddleEarthInhabitant, but he doesn’t have to dance.