Python conditions and boundary checks

Where we are on the map

This is lesson 30 of 72 in the Python block (27–38). The support map shows verifiable transitions. We continue the 0.3 paper contract: five reminder outcomes and the original fictional tasks.

Situation and question

The previous expression answered only yes or no. The assistant needs five outcomes: done, no date, overdue, remind, or not yet. How do we choose exactly one without losing an exceptional case? The paper contract made checking order important; in Python it becomes line order.

New words without gaps

A condition is an expression yielding True or False. Branching chooses a path from that answer. if checks first, elif checks another condition only if earlier ones failed, and else takes what remains. Lines at the same indentation form a block; in Python indentation is part of syntax, the language’s writing rules. A boundary case sits exactly on a threshold, such as two days. Mutually exclusive outcomes mean one task receives exactly one status.

The lesson’s support signal

Input → check the rule → change state → observable result. Point to where the program reads data, compares it with the contract, and merely reports the result. If a step is absent from the map, find it in the code and add it to your own trace.

Work through it step by step

Lay out cards −1, 0, 2, 3. Walk each through the sample branches: −1 stops at days < 0; 0 and 2 reach days <= 2; 3 falls to else. Now imagine done=True: the paper contract demands DONE before any date comparison. The short sample below omits done to isolate boundaries; put it first in the project function.

Predict and check

Cover the output block. Trace names line by line and write the exact predicted text, including case and line order. Only then run the code with python3 from step-03 and compare character by character. Change one input and predict again before running. If results differ, find the first divergent line instead of adjusting the prediction afterwards.

for days in (-1, 0, 2, 3):
    if days < 0:
        status = "OVERDUE"
    elif days <= 2:
        status = "REMIND"
    else:
        status = "NOT_YET"
    print(days, status)

Expected output

-1 OVERDUE
0 REMIND
2 REMIND
3 NOT_YET

Catch the error

If days <= 2 comes first, overdue −1 becomes REMIND. Four independent if statements may print several answers. Repair the order with an if/elif/else chain, and handle a missing date before comparing it with a number.

Project change

Turn the version 0.3 table into a function using return; do not print inside each branch. Leave display to the caller so the same function can later serve the CLI and tests.

Task and evidence

Write a complete function that takes done and days and returns one of the paper contract’s five outcomes. Before running it, draw a table for −1, 0, 1, 2, 3, None, and done=True. Give a partner only the code and test cards; they should obtain your answers without spoken help.

Transfer to a new setting

A ticket machine might return “paid,” “price missing,” “not enough money,” or “ready.” Which condition must come first if the purchase is already paid? Explain how that resembles done=True.

Return after 1, 7, and 30 days

After 1 day, recall the rule and one boundary case without this page. After 7 days, explain a new error example to a classmate. After 30 days, rerun the project test, check earlier records and output still match, and transfer the rule to a different task again.

Next lesson

Continue: Python loops that know when to stop