for Loops
Repeat once for each item
In this lesson
Explain it like I’m 5
‘For each thing in this pile, do something with it.’ Python hands you the items one at a time so you can deal with each one.
Looping over a collection
The pattern is for item in collection:. On each pass, the loop variable (here item) is automatically set to the next element. You can loop over lists, strings (character by character), dictionaries, and more — anything Python calls iterable.
fruits = ["apple", "pear", "kiwi"]
for fruit in fruits:
print("I like", fruit)
I like apple I like pear I like kiwi
Counting with range()
When you want to repeat a fixed number of times, use range(). range(5) produces 0, 1, 2, 3, 4 (it starts at 0 and stops before the end). range(1, 6) gives 1–5, and range(0, 10, 2) counts by twos. range is the idiomatic way to say ‘do this N times’.
total = 0
for n in range(1, 6):
total = total + n
print("Sum 1..5 is", total)
Sum 1..5 is 15
Write a loop that prints the 3-times table from 3×1 to 3×10. Run it and check the last line is 3 x 10 = 30.
for n in range(1, 11):
print("3 x", n, "=", 3 * n)To reach 10, your range must stop at 11: range(1, 11).
for n in range(1, 11):
print("3 x", n, "=", 3 * n)Knowing the position with enumerate()
Sometimes you need both the item and its index. Rather than managing a counter by hand, wrap the collection in enumerate(): for index, item in enumerate(names): gives you both each pass. It’s cleaner and less error-prone than tracking the index yourself.
fruits = ["apple", "pear", "kiwi"]
for index, fruit in enumerate(fruits):
print(index, fruit)
0 apple 1 pear 2 kiwi
Nested loops
A loop can contain another loop. For every pass of the outer loop, the inner loop runs completely. This is how you process grids, tables, or all pairs of things — but each level of nesting multiplies the work, so keep an eye on how many total iterations you’re creating.
for row in range(1, 3):
for col in range(1, 4):
print(row, col)
1 1 1 2 1 3 2 1 2 2 2 3
Looping over two lists with zip(), and reversed()
Two built-ins make for loops more expressive. zip(a, b) walks two (or more) lists in parallel, handing you one item from each on every pass — perfect when related data lives in separate lists. reversed(seq) walks a sequence back to front.
(When your goal is to transform a list into a new one rather than just visit it, a list comprehension is often cleaner — that has its own lesson.)
names = ["Ada", "Bo", "Cy"]
scores = [90, 75, 88]
for name, score in zip(names, scores):
print(name, "scored", score)
for n in reversed([1, 2, 3]):
print(n)
Ada scored 90 Bo scored 75 Cy scored 88 3 2 1
Common mistake: Off-by-one errors with range()
range excludes its stop value, so range(1, 5) gives 1–4, not 1–5. It’s easy to forget the end is not included.
Remember: to include N, write range(start, N + 1). When in doubt, print the range as a list to see exactly what it produces.
Common mistake: Modifying a list while looping over it
Adding or removing items from the same list you’re iterating confuses Python’s internal position tracker and skips or repeats items.
Loop over a copy (for x in items[:]:) or build a new list instead of changing the one you’re iterating.
What numbers does range(2, 7) produce?
range starts at the first value and stops before the second, so 7 is excluded.
Why use enumerate() in a for loop?
enumerate() hands you the index and the item together, so you don’t track a counter yourself.
Mini exercise (medium)
Given words = ["red", "green", "blue"], print each word with its position starting at 1, like 1: red.
Do it here. Finish the code below and press Run to try your answer right in the browser — no setup needed.
words = ["red", "green", "blue"]
lines = []
# TODO: build "1: red", "2: green", "3: blue" (number from 1)
for word in words:
lines.append(word)
print(lines)Use enumerate(words, start=1) to begin counting at 1.
words = ["red", "green", "blue"]
lines = []
for i, word in enumerate(words, start=1):
lines.append(f"{i}: {word}")
print(lines)