Generation STEM
Learning LibraryCore Coding LibraryTeens

enumerate() in Python: the item and its position, together

"Day 6 was the best day." To say that, a loop needs both the value and which day it was. enumerate() hands you both at once.

The big idea

enumerate(sequence) yields pairs of an index and the item at that index, so a loop can use both without managing a counter.

See it in code

1The basics

Each pass receives two things, unpacked into two loop variables: the position, then the item:

python
daily_tees = [42, 57, 38]

for index, tees in enumerate(daily_tees):
    print(index, tees)
Run it. Position and value together:
0 42
1 57
2 38

The index starts at 0, matching list positions. No range, no len, and no bracket lookups.

2A step further

People count days from 1. Pass start=1 and the numbering shifts, while the items stay the same:

python
daily_tees = [42, 57, 38, 64, 71, 95, 88]

for day, tees in enumerate(daily_tees, start=1):
    print(f"Day {day}: {tees} tees")
Run it. A numbered sales report:
Day 1: 42 tees
Day 2: 57 tees
Day 3: 38 tees
Day 4: 64 tees
Day 5: 71 tees
Day 6: 95 tees
Day 7: 88 tees

start=1 changes only the number you're handed. It doesn't skip the first item or alter the list.

3At Evergreen

With the position available, a best-so-far search can report which day won, not just how much it sold:

python
daily_tees = [42, 57, 38, 64, 71, 95, 88]
price = 38

best_day = 0
best_tees = 0
for day, tees in enumerate(daily_tees, start=1):
    if tees > best_tees:
        best_tees = tees
        best_day = day
print(f"Best day: day {best_day}, with {best_tees} tees (${best_tees * price:,})")
Run it. The best day of launch week:
Best day: day 6, with 95 tees ($3,610)

max(daily_tees) would have found 95, but not that it happened on day 6. Whenever position carries meaning, loop with enumerate.

The same idea, everywhere

Position matters in plenty of data: the line number of an error in a file, the index of a base where two sequences differ, the rank of a search result.

Try it yourself

Print only the days that beat a target of 60, with their day numbers. Then loop over two lists at once by using the index from enumerate to read a second, parallel list.

The common mistake

Forgetting to unpack. for item in enumerate(daily_tees) gives you tuples like (0, 42), and arithmetic on them fails. Name both parts: for day, tees in enumerate(...).

What it unlocks

enumerate is a cleaner for loop for lists. The pairs it produces are tuples, unpacked on the spot.

Related concepts