Python Conditional Statements

If-Elif-Else: Handling Multiple Possibilities

The elif statement (short for "else if") allows us to check multiple conditions in sequence. It extends the binary nature of if-else to accommodate multiple pathways.

Think of it as a series of filters or checkpoints, where Python evaluates each condition in order until it finds one that's True. Once it finds a True condition, it executes that block and skips all the rest.

if condition1:
    # Execute if condition1 is True
    statements_for_condition1
elif condition2:
    # Execute if condition1 is False AND condition2 is True
    statements_for_condition2
elif condition3:
    # Execute if condition1 and condition2 are False AND condition3 is True
    statements_for_condition3
else:
    # Execute if all conditions are False
    statements_when_all_false

The else block is optional. If omitted and all conditions are False, nothing will execute.

Example: Grading System

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Your grade is: {grade}")
# Output: Your grade is: B

Note how we only need to specify the lower bound in each condition, because once a condition is satisfied, the subsequent conditions aren't checked. This is more efficient and readable than writing:

# Less elegant approach
if score >= 90:
    grade = "A"
elif score >= 80 and score < 90:
    grade = "B"
elif score >= 70 and score < 80:
    grade = "C"
# and so on...

Real-World Application: Multi-tier Pricing System

item_count = int(input("How many items are you purchasing? "))
base_price = 10.0

if item_count == 1:
    total = base_price
    print(f"Regular price: ${total:.2f}")
elif 2 <= item_count <= 5:
    total = item_count * base_price * 0.95  # 5% discount
    print(f"5% bulk discount applied: ${total:.2f}")
elif 6 <= item_count <= 10:
    total = item_count * base_price * 0.9  # 10% discount
    print(f"10% bulk discount applied: ${total:.2f}")
else:
    total = item_count * base_price * 0.8  # 20% discount
    print(f"20% bulk discount applied: ${total:.2f}")

Important Consideration: Order Matters!

With if-elif-else chains, the order of conditions can significantly impact behavior. Python evaluates from top to bottom and executes the first True condition it encounters.

Consider our grading example with the conditions reversed:

# Incorrect order
score = 85

if score >= 60:  # This will always trigger for passing scores
    grade = "D"
elif score >= 70:
    grade = "C"
elif score >= 80:
    grade = "B"
elif score >= 90:
    grade = "A"
else:
    grade = "F"

print(f"Your grade is: {grade}")
# Output: Your grade is: D (Incorrect!)

Since 85 satisfies the first condition (≥60), Python assigns "D" and skips all other checks—even though the score qualifies for a higher grade!