Customizing functionality via inheritance

Object-Oriented Programming in Python

Alex Yarosh

Content Quality Analyst @ DataCamp

a hierarchy of account classes

What we have so far

class BankAccount:
    def __init__(self, balance):
       self.balance = balance

    def withdraw(self, amount):
       self.balance -=amount

# Empty class inherited from BankAccount
class SavingsAccount(BankAccount):
    pass

Customizing constructors

class SavingsAccount(BankAccount):    

    # Constructor specifically for SavingsAccount with an additional parameter
    def __init__(self, balance, interest_rate):  

       # Call the parent constructor using ClassName.__init__()
       BankAccount.__init__(self, balance)  # <--- self is a SavingsAccount but also a BankAccount

       # Add more functionality
       self.interest_rate = interest_rate

Can run constructor of the parent class first by Parent.__init__(self, args...)
Add more functionality
Don't have to call the parent constructors

Create objects with a customized constructor

# Construct the object using the new constructor
acct = SavingsAccount(1000, 0.03)
acct.interest_rate

0.03

Adding functionality

Add methods as usual
Can use the data from both the parent and the child class

class SavingsAccount(BankAccount):    

  def __init__(self, balance, interest_rate): 
     BankAccount.__init__(self, balance) 
     self.interest_rate = interest_rate

  # New functionality
  def compute_interest(self, n_periods = 1):
     return self.balance * ( (1 + self.interest_rate) ** n_periods - 1)

a hierarchy of account classes

Customizing functionality

class CheckingAccount(BankAccount):

  def __init__(self, balance, limit):
    BankAccount.__init__(self, balance)
    self.limit = limit

  def deposit(self, amount):
    self.balance += amount

  def withdraw(self, amount, fee=0):

    if fee <= self.limit:
      BankAccount.withdraw(self, amount + fee)  
    else:
      BankAccount.withdraw(self, 
                           amount + self.limit)

Can change the signature (add parameters)
Use Parent.method(self, args...) to call a method from the parent class

check_acct = CheckingAccount(1000, 25)

# Will call withdraw from CheckingAccount
check_acct.withdraw(200)

# Will call withdraw from CheckingAccount
check_acct.withdraw(200, fee=15)

bank_acct = BankAccount(1000)

# Will call withdraw from BankAccount
bank_acct.withdraw(200)

# Will produce an error
bank_acct.withdraw(200, fee=15)

TypeError: withdraw() got an unexpected
 keyword argument 'fee'

Let's practice!

Object-Oriented Programming in Python