Last Updated: Saturday 19th October 2013

What is Python's range() Function?

As an experienced Python developer, or even a beginner, you've likely heard of the Python range() function. But what does it do? In a nutshell, it generates a list of numbers, which is generally used to iterate over with for loops. There's many use cases. Often you will want to use this when you want to perform an action X number of times, where you may or may not care about the index. Other times you may want to iterate over a list (or another iterable object), while being able to have the index available.

The range() function works a little bit differently between Python 2.x and 3.x under the hood, however the concept is the same. We'll get to that a bit later, however.

Python's range() Parameters

The range() function has two sets of parameters, as follows:

range(stop)

  • stop: The number of numbers (integers) to generate, starting from zero.

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

  • start: The starting number of the sequence.
  • stop: The number of numbers (integers) to generate, starting from zero.
  • step: The difference between each number in the sequence.

Note that:

  • All parameters must be integers.
  • All parameters can be positive or negative.
  • The stop parameter is not the number the function will stop on. It specifies that it will stop on the Nth number produced, where stop is the Nth number.

Python's range() Function Examples

Simple Usage

Iterating Lists

99 Bottles of Beer on the Wall...

With the following code:

We get the following output:

Brilliant! Finally you can see the true power of Python :). If you're a little confused, for reference see the Wikipedia article.

Python's range() vs xrange() Functions

You may have heard of a function known as xrange(). This is a function that is present in Python 2.x, however it was renamed to range() in Python 3.x, and the original range() function was deprecated in Python 3.x. So what's the difference? Well, in Python 2.x range() produced a list, and xrange() returned an iterator - a sequence object. We can see this in the following example:








So in Python 3.x, the range() function got its own type. In basic terms, if you want to use range() in a for loop, then you're good to go. However you can't use it purely as a list object. For example you cannot slice a range type.

When you're using an iterator, every loop of the for statement produces the next number on the fly. Whereas the original range() function produced all numbers instantaneously, before the for loop started executing. The problem with the original range() function was that it used a very large amount of memory when producing a lot of numbers. However it tends to be quicker with a small amount of numbers. Note that in Python 3.x, you can still produce a list by passing the generator returned to the list() function. As follows:





To see the speed difference between the original range() and xrange() function, you may want to check out this article.

Using floats with Python's range() function

Unfortunately the range() function doesn't support the float type. However don't get upset too soon! We can easily implement it with a function. There's several ways it can be done, however here's one.

Brilliant!

  • jerome

    “It specifies that it will stop on the Nth number produced, where stop is the Nth number.”
    Which is wrong, considering the examples.