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LESSON 271 OF 673 · TUPLES, SORTING & BUILT-INS

Enumerate and Zip in Python: Loop with a Count

Lesson Read the full explanation

TL ; DR:

enumerate counts the items while you loop. zip walks two lists side by side.

fruits = ["apple", "kiwi", "plum"]
prices = [3, 1, 2]

for number, fruit in enumerate(fruits, start=1):
    print(number, fruit)     # 1 apple, then 2 kiwi, then 3 plum

for fruit, price in zip(fruits, prices):
    print(fruit, price)      # apple 3, then kiwi 1, then plum 2

Full lesson:

You have looped over indexes with range(len(meals)). Then meals[i] gave the item. Python has a shorter way to get both.

enumerate(lst) gives each item of lst together with a count. The loop unpacks each pair into two names, as you did with tuples.

for i, meal in enumerate(meals):
    print(i, meal)

The count starts at 0. To start at 1, write enumerate(meals, start=1). People read numbered lists from 1.

zip(a, b) walks two lists side by side. It pairs the first item of a with the first item of b. Then it pairs the second items, and so on. Each pair is a tuple.

for fruit, price in zip(fruits, prices):
    print(fruit, price)

zip stops at the end of the shorter list. The extra items are skipped, with no error.

The two work together. enumerate(zip(fruits, prices), start=1) gives a count and a pair each time. Put parentheses around the pair's two names.

for number, (fruit, price) in enumerate(zip(fruits, prices), start=1):
    print(f"{number}. {fruit} costs {price}")

Without the parentheses, Python stops with a ValueError. Each item has two parts, not three.

Your task

A quiz show keeps its players in the list players. Their points are in scores, in the same order.

First, print the players as a numbered list from 1, like 1. Ana. Use enumerate.

Then print each player with their points, like Ana has 12 points. Use zip.

Expected output:

1. Ana
2. Ben
3. Cleo
Ana has 12 points
Ben has 9 points
Cleo has 15 points

STEP 1 OF 4

Guided

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PYTHON · TUPLES, SORTING & BUILT-INS

Enumerate And Zip in Python, explained

The problem with range(len(...))

To get both the position and the item, you can loop over indexes: for i in range(len(meals)) and then read meals[i]. It works, but Python has a shorter way to get both.

enumerate: items with a count

enumerate(lst) gives each item of lst together with a count. Each step produces a pair, and the loop unpacks it into two names, as you do with tuples.

meals = ["soup", "pasta"]
for i, meal in enumerate(meals):
    print(i, meal)

0 soup
1 pasta

The count starts at 0. To start at 1, write enumerate(meals, start=1), which is how people read numbered lists.

You can use enumerate on any sequence, strings included: for i, ch in enumerate("hi") gives 0 h, then 1 i. It is also the usual way to find where something is. For example, a loop with for i, s in enumerate(scores) and an if s == 15 can print the index 2 of the score 15 in [12, 9, 15], with no range(len(...)) in sight.

zip: two lists side by side

zip(a, b) pairs the first item of a with the first item of b, then the second items, and so on. Each pair is a tuple.

fruits = ["apple", "kiwi", "plum"]
prices = [3, 1, 2]

for fruit, price in zip(fruits, prices):
    print(fruit, price)

apple 3
kiwi 1
plum 2

zip stops at the end of the shorter list. The extra items are skipped without an error: list(zip([1, 2, 3], "ab")) is [(1, 'a'), (2, 'b')].

The two lists should match in length. When they don't, zip doesn't warn you: list(zip(["x", "y", "z"], [1, 2])) is [('x', 1), ('y', 2)], and the third name quietly vanishes. If you meet a result that is shorter than you expected, check the lengths first.

Zipped pairs are useful outside loops too: dict(zip(["a", "b"], [1, 2])) builds the dictionary {'a': 1, 'b': 2}, with the first list as the keys.

Using both together

enumerate(zip(fruits, prices), start=1) gives a count and a pair on every step, such as (1, ('apple', 3)). Put parentheses around the pair's two names when you unpack it:

for number, (fruit, price) in enumerate(zip(fruits, prices), start=1):
    print(f"{number}. {fruit} costs {price}")

1. apple costs 3
2. kiwi costs 1
3. plum costs 2

Without the parentheses, for number, fruit, price in ... stops with ValueError: not enough values to unpack (expected 3, got 2). Each item has two parts, a number and a pair, not three.

Reading the loop header

The names in a for line must match the shape of what the iterator produces. enumerate(players) produces a pair, so two names fit. zip(a, b) produces a pair, so two names fit. enumerate(zip(a, b)) produces a number and a pair, so you write two names, with the second one a pair in parentheses. When a loop fails with a "values to unpack" error, count the parts the iterator gives and compare them with the names you wrote.

The exercise, step by step

A quiz show keeps its players in players and their points in scores, in the same order.

Loop over enumerate(players, start=1) with the names number and player.
Inside, print number, a dot, a space and player, like 1. Ana, with an f-string. Without start=1 the first line would be 0. Ana.
Loop over zip(players, scores) with the names player and points.
Inside, print a line like Ana has 12 points. Each pass sees one player and that player's points, so no index is needed.

players = ["Ana", "Ben", "Cleo"]
scores = [12, 9, 15]

for number, player in enumerate(players, start=1):
    print(f"{number}. {player}")

for player, points in zip(players, scores):
    print(f"{player} has {points} points")

1. Ana
2. Ben
3. Cleo
Ana has 12 points
Ben has 9 points
Cleo has 15 points

Key takeaways

enumerate(lst, start=1) gives a count and an item; the count starts at 0 unless you set start.
zip(a, b) gives pairs, one item from each list, and stops at the shorter list.
To use both, write for number, (x, y) in enumerate(zip(a, b), start=1), keeping the parentheses.

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