Everything You Need to Know About the Python Range

Basic syntax for Python range()

The range() function follows this syntax:

range([start,] stop [, step])

The start, stop, and step parameters are all integers.

start and stop tell Python the numbers to start and stop generating integers at. The step parameter indicates whether numbers should be generated in sequence, or whether some specific integers should be skipped.

Note: only the stop parameter is required for the range() function to work. If you don’t provide a start value, Python will simply use zero.

To add one more number to the sequence, you need to raise the stop value by the amount of the step parameter. In the example below, the step value is not defined explicitly, so it's 1 by default. The current output is 0, 1, 2, 3.

Note: the stop value is not included in the generated range.

If you want the function to generate a 4 as well, increase the stop value from 4 to 5:

for i in range(4):
  print(i, end=', ')

0, 1, 2, 3,

Python range() only supports integers by default. If you want to create a sequence of float values, you’ll need to create a new version of the range()function:

def frange(start, stop, step):
  i = start
  while i < stop:
    yield i
    i += step

for i in frange(0.5, 1.0, 0.1):
  print(i)

0.5
0.6
0.7
0.7999999999999999
0.8999999999999999
0.9999999999999999

With this new function, you can create ranges of floating values.

Note: you can use other functions, such as len(), to specify the start, stop, or step parameters based on the length or other characteristics of another Python object.

Understanding the step parameter

The step parameter is used to specify the numeric distance between each integer generated in the range sequence. By default, range in Python uses a step value of 1.

If you want to generate a range that increases in increments greater than 1, you’ll need to specify the step parameter. For example, to generate a range from 0 to 10 by increments of 2, you would use:

range(0, 10, 2)

The step parameter can also be used to generate a sequence that counts down rather than up. In that case, you need to reverse the start and stop values as well as specify a negative number for the step parameter:

range(10, 0, -2)

Depending on your start, stop, and step values, it is also possible to generate a sequence that goes from negative to positive integers or from positive to negative integers:

range(-5, 5, 1)

range(4, -2, -2)

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Making a list of integers

Lists are extremely useful containers for data of different types in Python. How do you make a list of integers using range() in Python 3? Simply combine the list() command with the range() command:

print(list(range(10)))

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Using Python range() in a for loop

One of the most common uses of the range() function is for iterating over a series of values in a for loop. This is particularly useful if you want to access each of the values in a list or array, or, for example, only every other value.

To give an example of how range() works with for loops, let’s make a list of integers and then double every value in the list:

samples = [1, 3, 5, 7, 9]
for i in range(len(samples)):
  print("Element index[", i, "]", "Previous value ", samples[i], "Now ", samples[i] * 2)

Element index[ 0 ] Previous value  1 Now  2
Element index[ 1 ] Previous value  3 Now  6
Element index[ 2 ] Previous value  5 Now  10
Element index[ 3 ] Previous value  7 Now  14
Element index[ 4 ] Previous value  9 Now  18

In this example, the range() function is generating a sequence from 0 to 4. The for loop is then using each value generated by range(), one at a time, to access a value from the sampleList.

This brings up an important point about the way range() differs in Python 2 and Python 3. In Python 3, the range() function generates each number in the sequence one at a time. That means it iterates through the sequence it is generating, rather than generating a list of integers at once. In Python 2, the range() function generated a list of integers automatically.