Programming I · Branches and tests

Use comparisons to select a branch, trace a function, and choose tests that expose boundary errors. Start by reviewing variables and numeric comparisons.

Lesson and summary

Python evaluates the if condition first and checks each elif only if earlier conditions are false. The first true branch runs. else handles the remaining inputs. A return sends a value back to the caller and ends the current function call.

Worked example

def classify(t):
    if t < 0:
        return "cold"
    elif t < 100:
        return "moderate"
    else:
        return "hot"

For t = −1 the first condition succeeds. At 0 it fails and the second succeeds. At 100 both fail. The tests −1, 0, 99 and 100 distinguish the intended behavior on both sides of each threshold. Changing t < 100 to t ≤ 100 would make the test at 100 fail.

Common mistake

Checking a broad condition first can make a later branch unreachable. For instance, t < 100 would catch all negative values before a later t < 0 check.

Practice

Trace the output for −3, 0, 70 and 101. Write one assertion that specifically detects a mistake at 100.

Mini-exam · 10 points

A pump is off below 20, idle from 20 up to but excluding 80, and active from 80 upward. Write its classification function (6 points). Give tests on both sides of each threshold with expected outputs (4 points).

MIT OCW · Programming in Python