Loops

Loops let you repeat work without copy-pasting code. Python has two: for, which repeats a fixed number of times (or once per item in a collection), and while, which repeats as long as a condition stays true.

for loops and range()

range(n) produces the numbers 0, 1, 2, ..., n-1 — n numbers total, starting at 0:

for i in range(5):
    print(i)
# 0 1 2 3 4

range(start, stop) starts somewhere other than 0, and still stops before stop:

for i in range(1, 6):
    print(i)
# 1 2 3 4 5

That "stops before, not at, stop" behavior trips up a lot of beginners — if you want to include n itself, you need range(1, n + 1).

range(start, stop, step) adds a step size:

for i in range(0, 10, 2):
    print(i)
# 0 2 4 6 8

Looping over a collection directly

You can loop over a string, list, or other collection directly, without range() at all:

for char in "abc":
    print(char)
# a b c

for word in ["the", "quick", "fox"]:
    print(word)
# the quick fox

This is almost always preferable to indexing with range(len(...)) when you don't actually need the index.

Accumulator pattern

The single most common loop pattern: start a variable at some initial value outside the loop, then update it on every iteration.

total = 0
for n in [1, 2, 3, 4]:
    total += n   # shorthand for total = total + n
# total is 10

You'll use this pattern — an accumulator that starts before the loop and updates inside it — constantly for the rest of this section.