enumerate()

enumerate()

What it is

enumerate(iterable, start=0) wraps any iterable and yields (index, value) pairs as you loop, so you get the position of each item without maintaining a manual counter variable.

Before this feature

fruits = ["apple", "banana", "cherry"]

i = 0
for fruit in fruits:
    print(i, fruit)
    i += 1

After this feature

fruits = ["apple", "banana", "cherry"]

for i, fruit in enumerate(fruits):
    print(i, fruit)
# 0 apple
# 1 banana
# 2 cherry

for i, fruit in enumerate(fruits, start=1):   # custom starting index
    print(i, fruit)
# 1 apple
# 2 banana
# 3 cherry

print(list(enumerate(fruits)))
# [(0, 'apple'), (1, 'banana'), (2, 'cherry')]

Why this is better

• No manual counter to forget updating: i += 1 is exactly the kind of off-by-one bug source that enumerate eliminates.
• Reads as intent: for i, fruit in enumerate(fruits) says “index and value” directly, versus reconstructing that meaning from a separate counter variable.
• Lazy, like range: enumerate is itself an iterator — it doesn’t build a list of all the pairs upfront unless you explicitly convert it with list(...).

Key notes / edge cases

• enumerate works on any iterable, not just lists — strings, generators, file objects (line number + line text is a common use: for lineno, line in enumerate(f, start=1)).
• The start parameter changes the first index emitted, not which items are iterated — enumerate(fruits, start=1) still visits all three fruits, just numbered from 1.
• Common mistake: calling enumerate(fruits.items()) on a dict — dicts already yield keys when iterated, so enumerate(d) gives (index, key), not (index, key, value); for indexed key/value pairs you’d need enumerate(d.items()).

Quick practice

1. What does list(enumerate(['a', 'b', 'c'])) return?

   Answer

   `[(0, 'a'), (1, 'b'), (2, 'c')]`.

2. How do you make enumerate start counting from 1 instead of 0?

   Answer

   `enumerate(iterable, start=1)`.

3. Does enumerate work on a generator, or only on lists?

   Answer

   Any iterable — generators, strings, file objects, etc., not just lists.