Designing for inheritance and polymorphism

Object-Oriented Programming in Python

Alex Yarosh

Content Quality Analyst @ DataCamp

Polymorphism

Using a unified interface to operate on objects of different classes

account classes hierarchy

All that matters is the interface

# Withdraw amount from each of accounts in list_of_accounts
def batch_withdraw(list_of_accounts, amount):
   for acct in list_of_accounts:
      acct.withdraw(amount)

b, c, s = BankAccount(1000), CheckingAccount(2000), SavingsAccount(3000)
batch_withdraw([b,c,s]) # <-- Will use BankAccount.withdraw(),
                              # then CheckingAccount.withdraw(),
                              # then SavingsAccount.withdraw()

batch_withdraw() doesn't need to check the object to know which withdraw() to call

Liskov substitution principle

Base class should be interchangeable with any of its subclasses without altering any properties of the program

Wherever BankAccount works, CheckingAccount should work as well

photo of Barbara Liskov

Liskov substitution principle

Base class should be interchangeable with any of its subclasses without altering any properties of the program

Syntactically

function signatures are compatible
- arguments, returned values

Semantically

the state of the object and the program remains consistent
- subclass method doesn't strengthen input conditions
- subclass method doesn't weaken output conditions
- no additional exceptions

Violating LSP

→ Syntactic incompatibility

BankAccount.withdraw() requires 1 parameter, but CheckingAccount.withdraw() requires 2

→ Subclass strengthening input conditions

BankAccount.withdraw() accepts any amount, but CheckingAccount.withdraw() assumes that the amount is limited

→ Subclass weakening output conditions

BankAccount.withdraw() can only leave a positive balance or cause an error, CheckingAccount.withdraw() can leave balance negative

Violating LSP

→ Changing additional attributes in subclass's method

→ Throwing additional exceptions in subclass's method

$$\text{\textbf{\Huge{No LSP -- No Inheritance}}}$$

Let's practice!

Object-Oriented Programming in Python