Python range() Function

The Python range() function gives you a sequence of whole numbers, like 0, 1, 2, 3, 4. Pair it with a for loop and you can repeat code an exact number of times, count backwards, or jump in steps. In this lesson you'll learn all three ways to call range() and the one rule that trips up almost every beginner.

What is the range() function in Python?

range() is a built-in function that produces a sequence of integers (whole numbers). It takes up to three arguments: range(start, stop, step). The sequence begins at start, moves by step, and ends just before stop.

for number in range(5):
    print(number)

0
1
2
3
4

Five numbers, starting at 0, and 5 itself is not included. Keep that in mind; it's the most important fact about range().

range(stop): counting from zero

With one argument, range(n) counts from 0 up to n - 1, which gives you exactly n numbers. It's the quickest way to say "do this n times":

for _ in range(3):
    print("Hip hip hooray!")

Hip hip hooray!
Hip hip hooray!
Hip hip hooray!

The underscore _ is a normal variable name. Programmers use it to say "I need the loop, but I don't care about the number."

range(start, stop): choosing where to begin

Give two arguments and the first one becomes the starting number. This is handy when counting from 1 feels more natural, like seat numbers:

for seat in range(1, 6):
    print(f"Seat {seat} is ready")

Seat 1 is ready
Seat 2 is ready
Seat 3 is ready
Seat 4 is ready
Seat 5 is ready

To get 1 to 5, you write range(1, 6): the stop value is always one past the last number you want.

range(start, stop, step): skipping numbers

The third argument, step, is the gap between numbers. It defaults to 1. Here's a "train every other day" plan for two weeks:

for day in range(1, 15, 2):
    print(f"Day {day}: workout")

Day 1: workout
Day 3: workout
Day 5: workout
Day 7: workout
Day 9: workout
Day 11: workout
Day 13: workout

The next value after 13 would be 15, which is not below the stop value 15, so the sequence ends. A step of 5 is great for things like timers or price tiers:

for minutes in range(0, 30, 5):
    print(minutes, end=" ")

0 5 10 15 20 25

How do you count backwards with range()?

Use a negative step. The start must now be larger than the stop, and again the stop value is left out:

for n in range(5, 0, -1):
    print(n)
print("Happy New Year!")

5
4
3
2
1
Happy New Year!

Notice the countdown ends at 1, not 0, because 0 is the stop value. To include 0, write range(5, -1, -1).

range() is lazy: it doesn't store every number

If you print a range directly, you don't see a list of numbers:

seats = range(1, 6)
print(seats)
print(type(seats))

range(1, 6)
<class 'range'>

A range is lazy: it only remembers its start, stop and step, and works out each number at the moment it's needed. That's why range(1_000_000_000) is created instantly and uses almost no memory, even though it describes a billion numbers.

Turning a range into a list

When you do want to see all the values, pass the range to list(). (You'll learn all about lists in Python lists.)

print(list(range(5)))
print(list(range(2, 11, 2)))
print(list(range(10, 0, -3)))

[0, 1, 2, 3, 4]
[2, 4, 6, 8, 10]
[10, 7, 4, 1]

Length and membership

Ranges work with len() and the in operator from membership operators, and both are instant because Python can calculate the answer instead of counting:

evens = range(0, 101, 2)
print(len(evens))
print(42 in evens)
print(7 in evens)

51
True
False

Quick reference: the three forms of range()

Real-world example: numbering a playlist

Here's a small program that prints a numbered tracklist and a total running time. The range runs from 1 to the number of tracks, inclusive:

track_count = 4
minutes_per_track = 3

for track in range(1, track_count + 1):
    print(f"Track {track} of {track_count}")

print(f"Total time: {track_count * minutes_per_track} minutes")

Track 1 of 4
Track 2 of 4
Track 3 of 4
Track 4 of 4
Total time: 12 minutes

Writing track_count + 1 as the stop is a common and clear way to say "up to and including track_count."

Common mistakes with range()

1. Expecting the stop value to be included (off-by-one)

for n in range(1, 10):
    print(n, end=" ")

1 2 3 4 5 6 7 8 9

If you wanted 1 to 10, use range(1, 11). An "off-by-one" error means your loop runs one time too many or too few, and with range() it almost always comes from forgetting that the stop is excluded.

2. Counting down without a negative step

print(list(range(10, 1)))

[]

No error, just an empty range, so a loop over it silently does nothing. To go down, add a negative step: range(10, 1, -1).

3. Using a step of zero

range(0, 10, 0)

ValueError: range() arg 3 must not be zero

4. Passing decimal numbers

for price in range(0.5, 3.0):
    print(price)

TypeError: 'float' object cannot be interpreted as an integer

range() only works with integers. If you need 0.5, 1.0, 1.5..., loop over whole numbers and divide: for n in range(1, 7): print(n / 2).

Practice exercise

olympic_years = range(2000, 2033, 4)

for year in olympic_years:
    print(year)
print(f"That's {len(olympic_years)} Games.")

2000
2004
2008
2012
2016
2020
2024
2028
2032
That's 9 Games.

What's next?

You can now control exactly how many times a loop runs. Next, learn how to change a loop's flow from the inside, stopping early or skipping a turn, with Python break, continue and pass.