Find the Median of a List in Python

When you work with numerical list in Python, you may need to apply aggregation like finding the average. Most common operations are built-in in Python and no extra-code is needed. That being said, there is no function or method to retrieve the median of a list, a mathematical operation that is quite common even if the concept is less known as the average.

In this tutorial, we’ll see how you can extract the median of a list.

Quick reminder: what is a median?

While the average of a list is obtained by dividing the sum of the element of a list by its length, the median is somehow tricker to get. It is the middle value, which means that:

• If the list has an odd number of elements, we just have to sort it and get the element in the middle
• If the list has an even number of elements, the median is interpolated by taking the average of the two middle values

Some examples:

[1,2,3,4,5] #odd: median is 3 - position 2 
[1,2,3,4,5,6] #even - median is (3+4)/2  
Code language: PHP (php)

Create our own function

Using the definition we used before, we can create our own function to extract a median from a list. The code is pretty straightforward but if you want to understand it completely, keep in mind that:

• While len() count the number of elements in a list, its index starts at 0. If you have 5 elements, the median is the element whose index is 2 (and not 3).
• You need to use integer to access a value of a list by its index. Float will raise an error.

my_list = [3,4,5,1,2]
my_other_list = [3,1,6,7,4,9]

def get_median(l):
    #sort list
    l.sort()
    #if the number of elements is even 
    if len(l) % 2 == 0:
        #get the two middle values
        value1 = l[int(len(l)/2)]
        value2 = l[int(len(l)/2-1)]
        #return the average 
        return (value1+value2)/2
    #if the number of elements is odd
    else:
        return l[int((len(l)+1)/2 - 1)]
        

print(get_median(my_list))
print(get_median(my_other_list))
Code language: Python (python)

Output:

3
5.0
Code language: CSS (css)

Great, but we need to create our own function. If you don’t want to, you have an alternative solution.

Use statistics to get the median of a list

The library statistics comes with advanced functions, one of them allowing us to calculate a median.

my_list = [3,4,5,1,2]
my_other_list = [3,1,6,7,4,9]

from statistics import median 
print(median(my_list))
print(median(my_other_list))
Code language: JavaScript (javascript)

Output:

3
5.0
Code language: CSS (css)