Python Sort List

Summary: in this tutorial, you’ll learn how to use the Python List sort() method to sort a list.

Introduction to the Python List sort() method

To sort a list, you use the sort() method:

list.sort()Code language: Python (python)

The sort() method sorts the original list in place. It means that the sort() method modifies the order of elements in the list.

By default, the sort() method sorts the elements of a list using the less-than operator (<). In other words, it places the lower elements before the higher ones.

To sort elements from higher to lower, you pass the reverse=True argument to the sort() method like this:

list.sort(reverse=True)Code language: Python (python)

Python List sort() method examples

Let’s take some examples of using the sort() method.

1) Using the Python List sort() method to sort a list of strings

If a list contains strings, the sort() method sorts the string elements alphabetically.

The following example uses the sort() method to sort the elements in the guests list alphabetically:

guests = ['James', 'Mary', 'John', 'Patricia', 'Robert', 'Jennifer']
guests.sort()

print(guests)Code language: Python (python)

Output:

['James', 'Jennifer', 'John', 'Mary', 'Patricia', 'Robert']Code language: Python (python)

And the following example uses the sort() method with the reverse=True argument to sort the elements in the guests list in the reverse alphabetical order:

guests = ['James', 'Mary', 'John', 'Patricia', 'Robert', 'Jennifer']
guests.sort(reverse=True)

print(guests)Code language: Python (python)

Output:

['Robert', 'Patricia', 'Mary', 'John', 'Jennifer', 'James']Code language: Python (python)

2) Using the Python List sort() method to sort a list of numbers

If a list contains numbers, the sort() method sorts the numbers from smallest to largest.

The following example uses the sort() method to sort numbers in the scores list from smallest to largest:

scores = [5, 7, 4, 6, 9, 8]
scores.sort()

print(scores)Code language: Python (python)

Output:

[4, 5, 6, 7, 8, 9]Code language: Python (python)

To sort numbers from the largest to smallest, you use the sort(reverse=True) like this:

scores = [5, 7, 4, 6, 9, 8]
scores.sort(reverse=True)

print(scores)Code language: Python (python)

Output:

[9, 8, 7, 6, 5, 4]Code language: Python (python)

3) Using the Python List sort() method to sort a list of tuples

Suppose that you have a list of tuples like this:

companies = [('Google', 2019, 134.81),
             ('Apple', 2019, 260.2),
             ('Facebook', 2019, 70.7)]Code language: Python (python)

And you want to sort the companies list by revenue from highest to lowest. To do it:

First, specify a sort key and pass it to the sort() method. To define a sort key, you create a function that accepts a tuple and returns the element that you want to sort by:

def sort_key(company):
    return company[2]Code language: Python (python)

This sort_key() function accepts a tuple called company and returns the third element.

Note that the company is a tuple e.g., ('Google', 2019, 134.81). And the company[2] references the revenue like 134.81 in this case.

Second, pass the sort_key function to the sort() method:

companies.sort(key=sort_key, reverse=True)Code language: Python (python)

The sort() method will use the value returned by the sort_key() function for the comparisons.

Note that you just pass the function name without the parentheses to the sort() method:

companies = [('Google', 2019, 134.81),
             ('Apple', 2019, 260.2),
             ('Facebook', 2019, 70.7)]


# define a sort key
def sort_key(company):
    return company[2]



# sort the companies by revenue
companies.sort(key=sort_key, reverse=True)

# show the sorted companies
print(companies)
Code language: Python (python)

Output:

[('Apple', 2019, 260.2), ('Google', 2019, 134.81), ('Facebook', 2019, 70.7)]Code language: Python (python)

Using lambda expression

To make it more concise, Python allows you to define a function without a name with the following syntax:

lambda arguments: expressionCode language: Python (python)

A function without a name is called an anonymous function. And this syntax is called a lambda expression.

Technically, it’s equivalent to the following function:

def name(arguments):
    return expressionCode language: Python (python)

The following example uses the lambda expression to sort the companies by revenue from low to high:

companies = [('Google', 2019, 134.81),
             ('Apple', 2019, 260.2),
             ('Facebook', 2019, 70.7)]

# sort the companies by revenue
companies.sort(key=lambda company: company[2])

# show the sorted companies
print(companies)Code language: Python (python)

Output:

[('Apple', 2019, 260.2), ('Google', 2019, 134.81), ('Facebook', 2019, 70.7)]Code language: Python (python)

Summary

• Use the Python List sort() method to sort a list in place.
• The sort() method sorts the string elements in alphabetical order and sorts the numeric elements from smallest to largest.
• Use the sort(reverse=True) to reverse the default sort order.