14.1 Conditionals

Before you start

Work through the earlier fundamentals chapters. Use a Python 3 interpreter and run each example locally.

Core concepts

An if branch runs when its condition is truthy. elif is checked only when earlier branches failed, and else covers the remaining cases. Indentation defines the body.

Empty collections, zero, None, and empty strings are falsy. Use is None when you specifically mean a missing value; a value of zero may still be valid. Compare values with ==, not is.

Worked example

cpu = 85
if cpu >= 90:
    print("critical")
elif cpu >= 70:
    print("warning")
else:
    print("healthy")

Mini lab

Classify usage values 0, 70, 90, and 100. Expected labels: healthy, warning, critical, critical. Reject values outside 0–100.

for cpu in [0, 70, 90, 100]:
    if not 0 <= cpu <= 100:
        raise ValueError("CPU must be between 0 and 100")
    if cpu >= 90:
        print("critical")
    elif cpu >= 70:
        print("warning")
    else:
        print("healthy")

Knowledge check

Why should the >= 90 branch come before >= 70?

Common mistake

Test values exactly on either side of a threshold. Avoid conditions that accidentally exclude zero.

Completion checklist

• Run the example and explain its output.
• Complete the mini lab without copying the solution.
• Test an edge case and explain how the code handles it.
• Answer the knowledge check in your own words.

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