Decisions with if, elif, and else

Unit ID: M04-U02 Estimated active time: 22-30 minutes

A branch runs conditionally

score = 8

if score >= 8:
    print("Ready to continue")

The colon begins a block. Indentation marks the code controlled by the condition. Consistent indentation is part of Python syntax.

Choose one exclusive result

score = 6

if score >= 8:
    result = "ready"
elif score >= 0:
    result = "review"
else:
    result = "invalid"

print(result)

Python checks branches from top to bottom and runs the first matching branch. The remaining branches are skipped.

Order from specific to fallback

This order is wrong:

if score >= 0:
    result = "recorded"
elif score >= 8:
    result = "ready"

A score of 9 matches the first branch, so the more specific second branch never runs.

Write the stronger threshold first:

if score >= 8:
    result = "ready"
elif score >= 0:
    result = "review"
else:
    result = "invalid"

Independent if statements are different

Use separate if statements when more than one action may apply:

hours = 45
reasons = []

if type(hours) is not int:
    reasons.append("hours must be an integer")

if type(hours) is int and not 0 <= hours <= 40:
    reasons.append("hours must be from 0 to 40")

The second condition protects the numeric comparison from a wrong type.

Practice

Create a temperature_c classification:

• above 30: hot;
• from 20 through 30: warm;
• from 10 through 19: cool;
• below 10: cold.

Test 31, 30, 20, 19, 10, and 9. Boundary tests reveal branch-order mistakes.

Takeaway

if runs a conditional block. elif and else create one exclusive path, while independent if statements allow several findings. Branch order is part of the logic. Next, we will repeat a step for every item in a collection.