Collections and control flow express policy

Control flow is where a program's policy becomes visible. A for loop says what is repeated, if says which cases differ, and match can name a finite set of shapes when that improves clarity. Lists preserve order, sets express membership, and dictionaries connect keys to values. Choosing a set for a list of allowed codes communicates that duplicates and order do not matter. Choosing a dictionary for rows keyed by an identifier communicates lookup intent.

Write the simplest loop that makes the decision inspectable. A comprehension is useful when it maps or filters without side effects. It becomes a liability when it contains nested conditions, I/O, or mutations. Check boundaries explicitly: empty input, one item, duplicate keys, and a value just outside a threshold.

Truthiness is convenient and sometimes ambiguous. An empty list, 0, and None are all false, but they mean different things in a business rule. If zero is a valid quantity, write quantity is None when testing absence. Use enumerate for positions, zip for parallel sequences, and collections.Counter when frequency is the actual question.

Micro-lab in the app

Predict, run, change or break, explain: change the threshold in this loop and account for every printed branch.

for score in [42, 68, 71, 39]:
    print('pass' if score >= 60 else 'review')