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

Inheritance allows us to build new classes out of old classes. It allows us to extend functionality defined in a parent class and create children classes that extend and compartmentalize different pieces of functionality.

Inheritance models natural hierarchies that we're used to thinking about in the world. We can define one general class to model something like an Phone and then inherit the methods and properties of the class to make new classes out of the first class, like IPhone and AndroidPhone.

When we say sub-classes, or child classes, inherit methods and properties from a parent class we mean the child class has access to all of the functionality of it's parent, and it can define it's own functionality on top of that.

When we define a class to represent a Phone we can add the basic properties and functionality that all phones have.

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

After we define what a Phone is we can create classes that inherit from the Phone class and add their own properties and functionality.

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

• iPhones have a unique unlock method that accepts a fingerprint
• iPhones have a unique set_fingerprint method that accepts a fingerprint
• Android phones have a unique set_keyboard method that accepts a keyboard

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

class IPhone(Phone):
  def __init__(self, phone_number):
    super().__init__(phone_number):
    self.fingerprint = None
    
  def set_fingerprint(self, fingerprint):
    self.fingerprint = fingerprint
    
  def unlock(self, fingerprint=None):
    if (fingerprint == self.fingerprint):
      print("Phone unlocked because no fingerprint has not been set.")
      
    if (fingerprint == self.fingerprint):
      print("Phone unlocked. Fingerprint matches.")
    else:
      print("Phone locked. Fingerprint doesn't match.")
  
class Android(Phone):
  def __init__(self, phone_number):
    super().__init__(phone_number)
    self.keyboard = "Default"
    
  def set_keyboard(self, keyboard):
    self.keyboard = keyboard

Inheritance Syntax

There's two new pieces of syntax used in the code above.

• Class definitions can accept a parameter specifying what class they inherit from.
• Child classes can invoke a method called super() to gain access to methods defined in the parent class and execute them.

Take another look at the Phone classes to see how these pieces of syntax are used to define how the classes define their inheritance and how the super() method is used.

Notice how the Android class doesn't repeat the code that attaches the phone_number passed to the __init__ method to the self reference. The Android class calls the parent constructor through the super method and allows the parent class to execute that default behavior.

class Parent:
  def __init__(self, phone_number):
    self.phone_number = phone_number
    
class Android(Phone):
  def __init__(self, phone_number):
    super().__init__(phone_number)

Exercise: Write Bank Account Classes

Let's practice writing classes and using inheritance by modelling different types of Bank accounts.

• Create a base BankAccount class
  • Bank accounts keep track of their current balance
  • Bank accounts have a deposit method
  • Bank accounts have a withdraw method
  • the deposit method returns the balance of the account after adding the deposited amount.
  • the withdraw method returns the amount of money that was successfully withdrawn.
  • Bank accounts return False if someone tries to deposit or withdraw a negative amount.
  • Bank accounts are created with a default interest rate of 2%
  • Bank accounts have a accumulate_interest method that sets the balance equal to the balance plus the balance times the interest rate
  • accumulate_interest returns the balance of the account after calculating the accumulated interest
• Create a ChildrensAccount class
  • Children's bank accounts have an interest rate of Zero.
  • Every time accumulate_interest is executed on a Child's account the account always gets $10 added to the balance.
• Create an OverdraftAccount class
  • An overdraft account penalizes customers for trying to draw too much money out of their account.
  • Overdraft accounts are created with an overdraft_penalty property that defaults to $40.
  • Customer's aren't allowed to withdraw more money than they have in their account. If a customer tries to withdraw more than they have then the withdraw method returns False and their balance is deducted only by the amount of the overdraft_penalty.
  • Overdraft accounts don't accumulate interest if their balance is below zero.

Sample Input:

basic_account = BankAccount()
basic_account.deposit(600)
print("Basic account has ${}".format(basic_account.balance))
basic_account.withdraw(17)
print("Basic account has ${}".format(basic_account.balance))
basic_account.accumulate_interest()
print("Basic account has ${}".format(basic_account.balance))
print()

childs_account = ChildrensAccount()
childs_account.deposit(34)
print("Child's account has ${}".format(childs_account.balance))
childs_account.withdraw(17)
print("Child's account has ${}".format(childs_account.balance))
childs_account.accumulate_interest()
print("Child's account has ${}".format(childs_account.balance))
print()

overdraft_account = OverdraftAccount()
overdraft_account.deposit(12)
print("Overdraft account has ${}".format(overdraft_account.balance))
overdraft_account.withdraw(17)
print("Overdraft account has ${}".format(overdraft_account.balance))
overdraft_account.accumulate_interest()
print("Overdraft account has ${}".format(overdraft_account.balance))

Sample Output:

Basic account has $600
Basic account has $583
Basic account has $594.66

Child's account has $34
Child's account has $17
Child's account has $27

Overdraft account has $12
Overdraft account has $-28
Overdraft account has $-28