Concepts / Understanding Python's range() Function

Understanding Python's range() Function

How It Works In this program, we are printing a sequence of numbers. We generate this sequence of numbers using the built-in range function.

  • Programming

What range() Does

The range() function is a built-in Python tool that generates a sequence of numbers. You use it most often inside a for loop to repeat a block of code a specific number of times. Instead of manually typing out every number you want to loop through, range() creates that sequence for you automatically. Think of it as a way to say 'give me the numbers from here to there' without having to write them all out by hand.

Key point

range() is a reusable piece of Python that works with any sequence. The for..in loop works for any sequence, so you can use range() to generate numbers, but you could also loop through lists, strings, or other sequences.

The Three Forms of range()

range() can be called in three different ways, each with a different number of arguments. Understanding which form to use depends on what sequence of numbers you need.

Definition

range(stop) — generates numbers starting from 0 up to (but not including) the stop value. For example, range(5) produces 0, 1, 2, 3, 4.

Definition

range(start, stop) — generates numbers starting from start up to (but not including) stop. For example, range(2, 7) produces 2, 3, 4, 5, 6.

Definition

range(start, stop, step) — generates numbers starting from start, moving by step each time, up to (but not including) stop. For example, range(0, 10, 2) produces 0, 2, 4, 6, 8.

range(stop)range(5)Starts at 00, 1, 2, 3, 4Stops before 5range(start, stop)range(2, 7)Starts at 22, 3, 4, 5, 6Stops before 7range(start, stop,step)range(0, 10, 2)Starts at 0, step by 20, 2, 4, 6, 8Stops before 10exampleproducesexampleproducesexampleproduces
How do range(n), range(start, stop), and range(start, stop, step) differ in what they output?

How Parameters Control the Sequence

Each parameter in range() has a specific job. Understanding what each one does helps you predict exactly which numbers will be generated.

Definition

start — the first number in the sequence. If you do not provide a start value, it defaults to 0.

Definition

stop — the boundary where the sequence ends. The sequence stops BEFORE reaching this value; it never includes the stop value itself.

Definition

step — how much to add to each number to get the next one. If you do not provide a step value, it defaults to 1. You can use negative step values to count backwards.

start = 2First number2Start herestop = 10Upper boundary (excluded)42 + 2step = 2Increment each time64 + 286 + 210Stop before thisbeginsadd stepadd stepadd stepcannot reachboundary
What numbers does range(2, 10, 2) actually produce, and where does each parameter place boundaries?

The Exclusive Upper Bound

One of the most important things to remember about range() is that it stops BEFORE the stop value. This is called an exclusive upper bound. The stop value is a boundary that the sequence never reaches or includes.

What you mightexpect0, 1, 2, 3, 4, 5Includes the stop valueWhat range()actually does0, 1, 2, 3, 4Stops before the stop value
Why does range(5) give 0,1,2,3,4 and not include 5?
Key point

This design choice makes range() work well with zero-based indexing in Python. If you want numbers from 0 to 4, you write range(5). If you want numbers from 0 to 9, you write range(10). The stop value is always one more than the last number you want.

Using range() in a For Loop

The most common use of range() is inside a for loop. This lets you repeat a block of code a specific number of times, with a counter variable that changes each iteration.

Worked example

Printing Numbers 1 Through 5

Write a program that prints the numbers 1 through 5, each on its own line.

  1. 1
    Identify the sequence needed

    You want to print 1, 2, 3, 4, 5. This is five consecutive numbers starting at 1.

  2. 2
    Choose the right range() call

    To get numbers from 1 to 5, use range(1, 6). The start is 1, and the stop is 6 (one more than the last number you want).

  3. 3
    Write the for loop

    Use for i in range(1, 6): to loop through each number. The variable i will take on each value from the range, one at a time.

  4. 4
    Print inside the loop

    Inside the loop body, use print(i) to output each number.

Result

The program prints:

12345

python
Output (expected)
1
2
3
4
5

Counting Backwards with Negative Step

You can use a negative step value to make range() count backwards instead of forwards. When you do this, the start value should be larger than the stop value, and the sequence will decrease by the step amount each time.

Worked example

Counting Down from 5 to 1

Write a program that counts down from 5 to 1.

  1. 1
    Identify the direction

    You want to go backwards, so you need a negative step value.

  2. 2
    Set start and stop correctly

    Start at 5 (the highest number) and stop before 0 (so use 0 as the stop value). This ensures you get 5, 4, 3, 2, 1.

  3. 3
    Use a negative step

    Use step = -1 to decrease by 1 each time. The call is range(5, 0, -1).

Result

The program counts: 5, 4, 3, 2, 1

python
Output (expected)
5
4
3
2
1

Common Mistakes with range()

  • Common mistake

    Forgetting that range() stops before the stop value

    What it looks like
    range(5) produces 0, 1, 2, 3, 4 — not 0, 1, 2, 3, 4, 5
    Why it fails

    The stop value is an exclusive boundary. The sequence never includes it.

    Do this instead

    If you want numbers up to and including 5, use range(6) instead of range(5).

  • Common mistake

    Using the wrong order of arguments

    What it looks like
    Writing range(10, 1) when you meant range(1, 10)
    Why it fails

    With a positive step (the default), range(10, 1) produces an empty sequence because you cannot count up from 10 to 1.

    Do this instead

    Remember the order: range(start, stop, step). If start is larger than stop, use a negative step.

  • Common mistake

    Confusing the number of arguments with their meaning

    What it looks like
    Writing range(5, 10) thinking it means 'give me 5 numbers up to 10'
    Why it fails

    range(5, 10) means start at 5 and stop before 10, not 'start at 5 and give me 10 numbers'.

    Do this instead

    range(5, 10) produces 5, 6, 7, 8, 9 — five numbers, but because of the start and stop values, not because you asked for five.

  • Common mistake

    Forgetting to use range() inside a for loop

    What it looks like
    Writing print(range(5)) instead of looping through it
    Why it fails

    print(range(5)) prints the range object itself, not the individual numbers.

    Do this instead

    Use a for loop: for i in range(5): print(i) to print each number.

  • Common mistake

    Using a step of 0

    What it looks like
    range(0, 10, 0)
    Why it fails

    A step of 0 would create an infinite loop because the value never changes, so Python raises an error instead.

    Do this instead

    Always use a non-zero step value, typically 1 or -1.

Practice: Predicting range() Output

What do you think happens?

What numbers does range(3, 12, 3) produce?

  • 3, 6, 9, 12

  • 3, 6, 9

  • 0, 3, 6, 9, 12

  • 3, 4, 5, 6, 7, 8, 9, 10, 11

Reveal answer
Answer:

3, 6, 9

Starting at 3, stepping by 3 each time, and stopping before 12 gives you 3, then 6 (3+3), then 9 (6+3). The next value would be 12, but 12 is the stop boundary, so it is not included.

What do you think happens?

What numbers does range(10, 0, -2) produce?

  • 10, 8, 6, 4, 2, 0

  • 10, 8, 6, 4, 2

  • 10, 9, 8, 7, 6, 5, 4, 3, 2, 1

  • Empty sequence (no numbers)

Reveal answer
Answer:

10, 8, 6, 4, 2

Starting at 10 and stepping backwards by 2 each time, stopping before 0, gives you 10, 8, 6, 4, 2. The next value would be 0, but 0 is the stop boundary, so it is not included.

Key Takeaways

  1. 1
    range() generates a sequence of numbers. You can call it with one argument range(stop), two arguments range(start, stop), or three arguments range(start, stop, step).
  2. 2
    The stop value is always excluded from the sequence. range(5) gives 0, 1, 2, 3, 4 — never 5.
  3. 3
    If you do not provide a start value, it defaults to 0. If you do not provide a step value, it defaults to 1.
  4. 4
    Use range() inside a for loop to repeat code a specific number of times, with a counter variable that changes each iteration.
  5. 5
    You can use a negative step to count backwards. For example, range(5, 0, -1) counts down from 5 to 1.

At a glance

  • range() is a built-in function that generates sequences of numbers, most commonly used in for loops to repeat code a specific number of times.
  • range() has three forms: range(stop), range(start, stop), and range(start, stop, step), each controlling which numbers are generated.
  • The stop value is always excluded — range() stops before reaching it, which aligns with Python's zero-based indexing.
  • You can count backwards by using a negative step value, such as range(10, 0, -1) to count down from 10 to 1.
  • Common mistakes include forgetting the exclusive upper bound, confusing argument order, and trying to print range() directly instead of looping through it.

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