How To Reverse List In Python: 4 Techniques

Very often I get people asking me to write a post on how to reverse a list in python. This is because this question often comes up in interviews. So, to oblige them, I have decided to write on four good and tested ways you can reverse a list in python. I also show their timing so that you can choose the best for your needs.

 

The built-in python reversed function

The syntax of the built-in python reversed function is reversed(seq). You can see that it takes in any sequence as argument. Sequences are lists, strings, tuples etc. For a refresher on sequences, see my post on iterables. The function returns an iterator. Remember that an iterator is an object that has elements but in order to extract the elements you need to cast it to list or call the next method. Most times, you cast it to a list to get out the elements. But casting afterwards to a list for this method of reversing could be an overhead cost for the method although it is easy, and uses substantially less memory unless you are casting. This method is ideal for times when you are dealing with very large lists and just want to use the elements of the reversed list when needed rather than using all at once. In this instance, you would need to use a python for loop.

Let’s take an example:

You can see from the above that I had to cast the iterator from the python reversed function to a list to get out the results. That could be an overhead as we’ll see later.

The python slice technique

Slicing a sequence is one of the ubiquitous techniques you can find with python lists and sequences. The syntax of slicing is [start:stop:step] where start is the index you want to start the slice, stop is where you want to stop the slice and step is the steps you want to take when iterating through the list to create the slice. To reverse a list using the python slice technique, you just need to use this statement: list_name[::-1], which is a shorthand for saying copy the list and then walk through it backwards.

Here is an example:

The advantage of this technique is that it is adaptable for any sequence and not just for lists. Some people claim that it is not readable but I find that argument obscure. Slicing is common in python even for the beginner. The only disadvantage I see with the python slice technique is that it uses up memory if you have a large list. This is because to create the reversed list, it needs to copy the original list and then reverse it. This sequence takes up a large chunk of memory. But when you want the original list to remain unchanged, this technique is good and recommended.

The python list reverse method

The python list reverse method is a method of the list class. The syntax is list.reverse(). In my opinion, it seems to be the easiest since it is built for lists and seems to be the fastest so far. But we will consider that in the timing section below. Unlike in the built-in python reversed function, it does not create any overhead and unlike the slicing technique, it does not require large chunk of memory even when working with large lists. It is more optimized for reversing python lists.

The advantageous fact about it is that it reverses in place. But if you want to make use of the original list after reversing, then this technique is not for you.

Here is an example:

I usually use this technique whenever I am reversing lists but if I need the original, I use the slice technique. Just to make you know.

Reverse list programmatically by swapping items

Now the last method you can use is to reverse the list programmatically by swapping items in place. You can write your own code that iterates through the elements of the list and swaps the elements in place. Here is a sample code:

This code can run fast but is not optimized for large lists. It swaps the elements in place and modifies the original list. It is worse than all the python built-in methods.

Timing the methods.

Most times when we are dealing with large lists, we want something that works very fast and doesn’t use much memory. Although with the built-in timing functions in python we cannot calculate for memory usage, but we can find out how long it takes each of the techniques above to run. We will need to import the timeit module to do this.

Here is a sample code for all three built-in methods except the programmed reverse list swapping method. The swapping technique takes a longer time for large lists that is why it is excluded.

When you run the code above, you will see that the list reverse method takes the shortest time of all three methods. Overall, its running time is 12 times lesser than the reversed method. The reversed function took longer time because of the list casting overhead. If we had a for loop, it would have taken less time. The slicing technique comes second place. So, that is why I use the python list reverse method often when reversing lists.

The list reverse method works better because it has been optimized by the developers of python for lists. I believe they look to these things that is why it was made to be a method of the list class.

So, now you have the options you can choose from while reversing lists. Use any to your heart’s desire.

Happy pythoning.