for Loops and range()

The for loop in Python iterates over the items of any sequence.

words = ['cat', 'window', 'defenestrate']
for w in words:
    print(w)

cat
window
defenestrate

The range() Function

If you do need to iterate over a sequence of numbers, the built-in function, range() is the tool you need.

for i in range(3):
    print(i)

0
1
2

Remember: The given end point is never part of the generated sequence. Thus, range(3) yields 0, 1, 2 but not 3.

Start, Stop, and Step

You can also choose where to start and how big each step is. The full form is range(start, stop, step) — start counting at start, stop before stop, and jump by step each time.

for i in range(2, 8, 2):
    print(i)

2
4
6

Try It Yourself

Drag the slider to change how many rows this loop prints, then press Run. Notice that range(n) always starts at 0, so n is exactly how many times the loop body runs.

n = 5
for i in range(n):
    print('*' * (i + 1))

*
**
***
****
*****

Counting Backwards

You can even use a negative step to count backwards:

for i in range(0, -3, -1):
    print(i)

0
-1
-2

Iterating with Indices

If you need both the index and the item, you can iterate over the indices of a sequence:

a = ['Mary', 'had', 'a', 'little', 'lamb']
for i in range(len(a)):
    print(i, a[i])

0 Mary
1 had
2 a
3 little
4 lamb

Common Misconceptions

A for loop in Python is like C where you define the initialization, condition, and increment. Python’s for loop iterates over the items of any sequence in the order they appear.

range(5) generates numbers from 1 to 5. range(5) generates numbers from 0 to 4 (the end point is exclusive).

Edge Cases

• Empty sequences -> The loop body never executes.
• range() with start equal to or greater than stop (with positive step) -> Generates empty sequence.

Under the Hood

Watch how the for loop dynamically assigns the next value from range() to the iteration variable on each cycle:

for i in range(2, 8, 2):
    print("Value:", i)
print("Loop finished")