zip, enumerate & ranking

Pair and number things

Lesson 7 of 20 · 9 min · Module 2: Work with data efficiently

Data often arrives as parallel lists: one of names, one of scores. zip pairs them up and enumerate numbers them, so you never juggle indexes by hand.

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By the end, you’ll be able to…

Pair parallel lists with zip
Number items with enumerate
Sort stably by one field

The idea

zip(a, b) yields pairs (a[0], b[0]), (a[1], b[1])… and stops at the shorter list; zip(a, b, strict=True) raises ValueError when lengths differ. enumerate(items, start=1) yields (1, first), (2, second)…

names = ["Ana", "Bob"]
scores = [91, 78]
for rank, (name, score) in enumerate(zip(names, scores), start=1):
    print(rank, name, score)

01zip stops early by default

zip(["a", "b", "c"], [1, 2]) quietly drops "c". With strict=True (Python 3.10+), a length mismatch raises ValueError, which turns a hidden data bug into a clear error.

print(list(zip(["a", "b"], [1, 2])))

Output

[('a', 1), ('b', 2)]

02enumerate replaces range(len(...))

for i, item in enumerate(items, start=1) is clearer and less error-prone than indexing with range(len(items)).

03Stable sorting

Python’s sort never reorders items whose keys are equal, even with reverse=True. That gives fair, predictable tie handling for free.

Your turn

Write leaderboard(names, scores) that returns lines like "1. Mia (95)", highest score first; equal scores keep the order of the input. Raise ValueError if the lists have different lengths.

What the checks look for

Ranks names by score
Keeps input order for equal scores
Handles empty lists
Rejects lists of different lengths

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Take this with you

zip pairs, enumerate numbers, and a key decides how things are ordered.

Go a little further

Give equal scores the same rank (1, 2, 2, 4), as in sports tables.