Python Tip: zip

Loop over related lists together

Learn how to use zip to loop over related lists together without manual indexing.

Author

Shep Bryan IV

Python Tip: `zip`

Need to loop over two lists at the same time?

Use zip() to pair related values together.

Without `zip`

You can use indexes to read from two lists, but it makes the loop harder to read.

names = ["Ada", "Grace", "Katherine"]
scores = [98, 95, 99]

for i in range(len(names)):
    print(names[i], scores[i])

Ada 98
Grace 95
Katherine 99

With `zip`

zip() pairs the first item from each list, then the second, then the third.

names = ["Ada", "Grace", "Katherine"]
scores = [98, 95, 99]

for name, score in zip(names, scores):
    print(name, score)

Ada 98
Grace 95
Katherine 99

Cleaner loop. Same result.

More Than Two Lists

You can zip three or more lists when each item has multiple related values.

names = ["Ada", "Grace", "Katherine"]
scores = [98, 95, 99]
grades = ["A", "A", "A"]

for name, score, grade in zip(names, scores, grades):
    print(f"{name}: {score} ({grade})")

Ada: 98 (A)
Grace: 95 (A)
Katherine: 99 (A)

Watch the Lengths

zip() stops when the shortest list runs out of items.

names = ["Ada", "Grace", "Katherine"]
scores = [98, 95]

for name, score in zip(names, scores):
    print(name, score)

Ada 98
Grace 95

Katherine is skipped because there is no matching score.

Combine with `enumerate`

Use enumerate() with zip() when you also need a counter.

names = ["Ada", "Grace", "Katherine"]
scores = [98, 95, 99]

for rank, (name, score) in enumerate(zip(names, scores), start=1):
    print(f"{rank}. {name}: {score}")

1. Ada: 98
2. Grace: 95
3. Katherine: 99

Wrap-Up

Now you can use zip() to loop over related lists together.

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Shep Bryan IV