The range() Function

The range() Function#

Python’s range() produces a sequence of integers on demand. Paired with for, it covers every counter-based loop pattern.

One-Argument Form: range(n)#

range(n)   →  0, 1, 2, ..., n-1

Run it live and change n:

>>> for i in range(4):
...     print(i)
...
0
1
2
3

To see the full sequence at once, convert it to a list:

>>> print(list(range(4)))
[0, 1, 2, 3]

Two-Argument Form: range(start, stop)#

range(start, stop)   →  start, start+1, ..., stop-1

Run it live:

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

Note that stop is exclusive — the loop runs while i < stop.

Three-Argument Form: range(start, stop, step)#

range(start, stop, step)   →  start, start+step, start+2*step, ...

Counting by fives from 0 to 20. Run it and change the step:

>>> print(list(range(0, 25, 5)))
[0, 5, 10, 15, 20]

The sequence stops before it would equal or exceed stop.

The boundaries of range() are the classic source of off-by-one bugs. Change start, stop, and step and watch which integers appear:

>>> print(list(range(5)))           # 0 .. 4
[0, 1, 2, 3, 4]
>>> print(list(range(1, 6)))        # 1 .. 5  (stop is exclusive)
[1, 2, 3, 4, 5]
>>> print(list(range(0, 25, 5)))    # count by fives
[0, 5, 10, 15, 20]
>>> print(list(range(5, 0, -1)))    # count down
[5, 4, 3, 2, 1]
>>> for i in range(1, 6):
...     print(i, end=" ")
...
1 2 3 4 5

Reverse Iteration#

A negative step counts downward. Run it live:

>>> for i in range(5, 0, -1):
...     print(i)
...
5
4
3
2
1

range(n-1, -1, -1) visits indices n-1 down to 0.

An alternative that reads more naturally is reversed(range(n)):

>>> for i in reversed(range(5)):
...     print(i)
...
4
3
2
1
0

Range Objects Are Lazy#

range() does not build a list in memory — it computes each integer on demand. This makes range(1_000_000) just as cheap to create as range(5). Only use list(range(...)) when you actually need a list.

Summary#

Pattern

Python range() form

Count from 0 to n-1

for i in range(n):

Count from a to b-1

for i in range(a, b):

Count from a to b-1 by k

for i in range(a, b, k):

Count down from n-1 to 0

for i in range(n-1, -1, -1):