Nested if Statements

Note

Source: Adapted from the C# edition (decisions/ifnested.rst). Python-specific indentation guidance is original. Loan-approval and shipping-cost examples replace the original C# examples.

An if body can itself contain another if statement. This is called nesting. Python’s indentation makes the nesting level visually obvious.

Example: Loan Approval

A simple loan approval process requires both a minimum credit score and sufficient income:

credit_score = int(input("Credit score: "))
annual_income = float(input("Annual income: $"))

if credit_score >= 650:
    if annual_income >= 30000:
        print("Loan approved.")
    else:
        print("Denied: income too low.")
else:
    print("Denied: credit score too low.")

The outer if checks the credit score; only if that passes does the inner if check the income.

Reading Nested Code

Indentation levels tell you which else belongs to which if:

if a:
    if b:
        print("both a and b")
    else:
        print("a but not b")
else:
    print("not a")

The else at indentation level 4 belongs to if b:. The else at indentation level 0 belongs to if a:.

Flattening Nested if With and

When nesting exists only to combine conditions, you can often flatten it using and:

if credit_score >= 650 and annual_income >= 30000:
    print("Loan approved.")
else:
    print("Denied.")

This is simpler when you do not need separate messages for each failure reason.

Example: Shipping Cost

Shipping cost depends on both weight and destination zone:

def shipping_cost(weight, zone):
    """Return shipping cost based on weight and zone (1 or 2)."""
    if zone == 1:
        if weight <= 5:
            return 3.99
        else:
            return 3.99 + 0.50 * (weight - 5)
    else:   # zone == 2
        if weight <= 5:
            return 6.99
        else:
            return 6.99 + 0.75 * (weight - 5)

Nesting is appropriate here because the cost formula differs by zone.

Avoiding Excessive Nesting

Deep nesting (three or more levels) is hard to read. Consider these refactoring options:

• Extract nested logic into a helper function.

• Use and / or to combine conditions.

• Use elif to flatten a chain of nested if/else statements.

# deep nesting — harder to follow
if a:
    if b:
        if c:
            do_thing()

# flattened with and — clearer
if a and b and c:
    do_thing()