The Dependency Inversion Principle: High level modules should not depend on low level modules.

dependency_inversion_principle.py

# Dependency Inverson Principle
# High level modules should not depend on low level modules
# Without Dependency Inversion:
class LightBulb_wo:
    def turn_on(self):
        print("LightBulb: turned on...")

    def turn_off(self):
        print("LightBulb: turned off...")


class ElectricPowerSwitch_wo:
    def __init__(self, l: LightBulb_wo):
        self.lightbulb = l
        self.on = False

    def press(self):
        if self.on:
            self.lightbulb.turn_off()
            self.on = False
        else:
            self.lightbulb.turn_on()
            self.on = True


lightBulb = LightBulb_wo()
switch = ElectricPowerSwitch_wo(lightBulb)
switch.press()
switch.press()

# ElectricPowerSwitch depends on LightBulb
# What if we want to add a fan?

# With Dependency Inversion:
from abc import ABC, abstractmethod


class Switchable(ABC):
    @abstractmethod
    def turn_on(self):
        pass

    @abstractmethod
    def turn_off(self):
        pass


class LightBulb(Switchable):
    def turn_on(self):
        print("LightBulb: turned on...")

    def turn_off(self):
        print("LightBulb: turned off...")


class Fan(Switchable):
    def turn_on(self):
        print("Fan: turned on...")

    def turn_off(self):
        print("Fan: turned off...")


class ElectricPowerSwitch:
    def __init__(self, c: Switchable):
        self.client = c
        self.on = False

    def press(self):
        if self.on:
            self.client.turn_off()
            self.on = False
        else:
            self.client.turn_on()
            self.on = True


lightBulb = LightBulb()
switch = ElectricPowerSwitch(lightBulb)
switch.press()
switch.press()

fan = Fan()
switch1 = ElectricPowerSwitch(fan)
switch1.press()
switch1.press()
# The key idea of the Dependency Inversion Principle is to depend upon abstractions, not concretions. 
# In our example, ElectricPowerSwitch (the high-level module) doesn't depend on LightBulb or Fan (the low-level modules). Instead, it depends on the Switchable interface. This allows us to add new Switchable objects without having to modify ElectricPowerSwitch at all. If we didn't have the Switchable interface and wanted to add a new device that can be switched on and off, we would have to modify ElectricPowerSwitch which would violate the Open/Closed principle.