python-class-inheritance.md

Python Class Inheritance

Inheritance in Python

Objectives

• Implement inheritance
• Describe what has been inherited from one class to another
• Identify inheritance syntax
• Use inheritance to reduce repetition
• Use inheritance to model the world

Review

1. What is a method?
2. What is a class?
3. What is an instance method?
4. Why do we use a class?

Inheritance

To avoid reinventing the wheel, we can write sub-classes aka child classes that inhehrit the properties of their super class (aka parent class). You've actually already seen this if you've written a class-based React component that starts with class Something extends React.Component. The componente (i.e. the class) you're writing inherits lots of properties from a superclass supplied by React, which has things like state, lifecycle methods, etc.

Super Class: Phone

Let's dive into an example. We'll a Phone class, then some sub-classes of Phone, namely IPhone and AndroidPhone.

All phones

• have a phone number
• can place phone calls
• can send text messages

Write a Phone class that has a number property, a __str__ method, as well as call and text methods:

class Phone:
    def __init__(self, phone_number):
        self.number = phone_number

    def __str__(self):
        return str(self.number)

    def call(self, other_number):
        print("Calling {} from {}.".format(self.number, other_number))

    def text(self, other_number, msg):
        print("Sending text from {} to {}:".format(self.number, other_number))
        print(msg)

1111111111
2222222222
Calling 1111111111 from 2222222222.
Sending text from 2222222222 to 1111111111:
hello, world

Try making some phone instances and calling their methods!

Sub-classes: Iphone & AndroidPhone

Let's define two new classes that inherit from the Phone class. We'll make an IPhone and an AndroidPhone.

Iphone

First thing we need when creating a sub-class is to pass it the parent class:

class IPhone(Phone):

Next, we call the parent constructor and pass any paremeters from the parent constructor into it:

class IPhone(Phone):
  def __init__(self, phone_number, generation, color):
    super().__init__(phone_number)
    self.generation = generation
    self.color = color

Create an instance of IPhone and verify that it inherited all the same functionality of the Phone class:

iPhone3 = IPhone(3333333333, '7s', 'black')
print(iPhone3)
iPhone3.call(phone2)
phone1.text(iPhone3, 'im so fancy')

3333333333
Calling 3333333333 from 2222222222.
Sending text from 1111111111 to 3333333333:
im so fancy

You can overwrite methods from the superclass by simple redefining it in the sub-class. For example, let's write a __str__ method for IPhone and see what happens:

class IPhone(Phone):
    def __init__(self, phone_number, generation, color):
        super().__init__(phone_number)
        self.generation = generation
        self.color = color

    def __str__(self):
        return(f'{self.generation} {self.color} Iphone {self.number}')

iPhone3 = IPhone(3333333333, '7s', 'black')

iPhones have that cool/scary fingerprint functionality now - let's make our IPhone class mimic this behavior, by adding a set_fingerprint method, as well as an unlock method.

1. Add a fingerprint property to the constructor that initializes to None
2. set_fingerprint: takes a fingerprint and sets it to the instance's fingerprint property
3. unlock: accepts a fingerprint and and compares it to the stored fingerprint for that instance. If there is no stored fingerprint, or if the input fingerprint doesn't match the stored fingerprint, do not unlock. Otherwise, unlock

class IPhone(Phone):
    def __init__(self, phone_number, generation, color):
        super().__init__(phone_number)
        self.generation = generation
        self.color = color
        self.fingerprint = None

    def __str__(self):
        return(f'{self.generation} {self.color} Iphone {self.number}')

        self.fingerprint = None

    def set_fingerprint(self, fingerprint):
        self.fingerprint = fingerprint

    def unlock(self, fingerprint=None):
        if (not self.fingerprint):
            print("Phone unlocked because the fingerprint has not been set.")
        elif (fingerprint == self.fingerprint):
            print("Phone unlocked. Fingerprint matches.")
        else:
            print("Phone locked. Fingerprint doesn't match.")

iPhone3 = IPhone(3333333333, '7s', 'black')
# print(iPhone3)
# iPhone3.call(phone2)
# phone1.text(iPhone3, 'im so fancy')
iPhone3.unlock()
iPhone3.set_fingerprint('Taylor\'s thumb')
iPhone3.unlock('Dave\'s thumb')
iPhone3.unlock('Taylor\'s thumb')

Phone unlocked because the fingerprint has not been set.
Phone locked. Fingerprint doesn't match.
Phone unlocked. Fingerprint matches.

Exercise: Android

Write an Android class that inherits from Phone.

• Each Android should have a keyboard property that is initialized to "Default"
• Add a set_keyboard method that accepts a keyboard
• Write a __str__ method that prints This Android uses the <insert keyboard here> keyboard.

Lab

You are now ready for the Bank Account Inheritance Lab