Python Nested If Statements: Hierarchical Conditional Logic

A nested if statement occurs when an if block or else block contains one or more inner if statements. Nested conditions are required when a secondary decision depends entirely on the outcome of a primary verification check (e.g. verifying an ATM PIN only after confirming that the account is active).

1. Syntax and Indentation of Nested If

if outer_condition:
    # Outer block (indented 4 spaces)
    if inner_condition:
        # Inner block (indented 8 spaces)
        execute_inner_statement
    else:
        execute_inner_fallback
else:
    execute_outer_fallback

2. Real-World Practical Example: Banking ATM Withdrawal

# Program: ATM Withdrawal Logic
account_active = True
account_balance = 15000.0
withdrawal_amount = 4000.0
daily_limit = 10000.0

if account_active:
    if withdrawal_amount <= daily_limit:
        if withdrawal_amount <= account_balance:
            account_balance -= withdrawal_amount
            print(f"Transaction Successful! Dispensing ₹{withdrawal_amount}.")
            print(f"Remaining Balance: ₹{account_balance}")
        else:
            print("Declined: Insufficient account balance!")
    else:
        print(f"Declined: Exceeds daily ATM limit of ₹{daily_limit}!")
else:
    print("Declined: Account is locked or inactive. Contact customer care.")

Refactoring Deep Nesting (The "Arrow Anti-Pattern")

When code is nested 4 or 5 levels deep, it forms a difficult-to-read triangle known as the arrow anti-pattern. You can flatten nested code using guard clauses or the and operator:

# Flattened using logical 'and':
if account_active and (withdrawal_amount <= daily_limit) and (withdrawal_amount <= account_balance):
    account_balance -= withdrawal_amount
    print("Withdrawal approved!")

Frequently Asked Questions (FAQ)