Python List sort()

prime_numbers = [11, 3, 7, 5, 2]

# sorting the list in ascending order
prime_numbers.sort()

print(prime_numbers)

# Output: [2, 3, 5, 7, 11]

sort() Syntax

The syntax of the sort() method is:

list.sort(key=..., reverse=...)

Alternatively, you can also use Python's built-in sorted() function for the same purpose.

sorted(list, key=..., reverse=...)

Note: The simplest difference between sort() and sorted() is: sort() changes the list directly and doesn't return any value, while sorted() doesn't change the list and returns the sorted list.

sort() Parameters

By default, sort() doesn't require any extra parameters. However, it has two optional parameters:

• reverse - If True, the sorted list is reversed (or sorted in Descending order)
• key - function that serves as a key for the sort comparison

sort() Return Value

The sort() method doesn't return any value. Rather, it changes the original list.

If you want a function to return the sorted list rather than change the original list, use sorted().

Example 1: Sort a given list

# vowels list
vowels = ['e', 'a', 'u', 'o', 'i']

# sort the vowels
vowels.sort()

# print vowels
print('Sorted list:', vowels)

Output

Sorted list: ['a', 'e', 'i', 'o', 'u']

Sort in Descending order

The sort() method accepts a reverse parameter as an optional argument.

Setting reverse = True sorts the list in the descending order.

list.sort(reverse=True)

Alternatively for sorted(), you can use the following code.

sorted(list, reverse=True)

Example 2: Sort the list in Descending order

# vowels list
vowels = ['e', 'a', 'u', 'o', 'i']

# sort the vowels
vowels.sort(reverse=True)

# print vowels
print('Sorted list (in Descending):', vowels)

Output

Sorted list (in Descending): ['u', 'o', 'i', 'e', 'a']

Sort with custom function using key

If you want your own implementation for sorting, the sort() method also accepts a key function as an optional parameter.

Based on the results of the key function, you can sort the given list.

list.sort(key=len)

Alternatively for sorted:

sorted(list, key=len)

Here, len is Python's in-built function to count the length of an element.

The list is sorted based on the length of each element, from lowest count to highest.

We know that a tuple is sorted using its first parameter by default. Let's look at how to customize the sort() method to sort using the second element.

Example 3: Sort the list using key

# take second element for sort
def takeSecond(elem):
    return elem[1]

# random list
random = [(2, 2), (3, 4), (4, 1), (1, 3)]

# sort list with key
random.sort(key=takeSecond)

# print list
print('Sorted list:', random)

Output

Sorted list: [(4, 1), (2, 2), (1, 3), (3, 4)]

Let's take another example. Suppose we have a list of information about the employees of an office where each element is a dictionary.

We can sort the list in the following way:

# sorting using custom key
employees = [
    {'Name': 'Alan Turing', 'age': 25, 'salary': 10000},
    {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000},
    {'Name': 'John Hopkins', 'age': 18, 'salary': 1000},
    {'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000},
]

# custom functions to get employee info
def get_name(employee):
    return employee.get('Name')


def get_age(employee):
    return employee.get('age')


def get_salary(employee):
    return employee.get('salary')


# sort by name (Ascending order)
employees.sort(key=get_name)
print(employees, end='\n\n')

# sort by Age (Ascending order)
employees.sort(key=get_age)
print(employees, end='\n\n')

# sort by salary (Descending order)
employees.sort(key=get_salary, reverse=True)
print(employees, end='\n\n')

Output

[{'Name': 'Alan Turing', 'age': 25, 'salary': 10000}, {'Name': 'John Hopkins', 'age': 18, 'salary': 1000}, {'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000}, {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000}]

[{'Name': 'John Hopkins', 'age': 18, 'salary': 1000}, {'Name': 'Alan Turing', 'age': 25, 'salary': 10000}, {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000}, {'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000}]

[{'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000}, {'Name': 'Alan Turing', 'age': 25, 'salary': 10000}, {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000}, {'Name': 'John Hopkins', 'age': 18, 'salary': 1000}]

Here, for the first case, our custom function returns the name of each employee. Since the name is a string, Python by default sorts it using the alphabetical order.

For the second case, age (int) is returned and is sorted in ascending order.

For the third case, the function returns the salary (int), and is sorted in the descending order using reverse = True.

It is a good practice to use the lambda function when the function can be summarized in one line. So, we can also write the above program as:

# sorting using custom key
employees = [
    {'Name': 'Alan Turing', 'age': 25, 'salary': 10000},
    {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000},
    {'Name': 'John Hopkins', 'age': 18, 'salary': 1000},
    {'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000},
]

# sort by name (Ascending order)
employees.sort(key=lambda x: x.get('Name'))
print(employees, end='\n\n')

# sort by Age (Ascending order)
employees.sort(key=lambda x: x.get('age'))
print(employees, end='\n\n')

# sort by salary (Descending order)
employees.sort(key=lambda x: x.get('salary'), reverse=True)
print(employees, end='\n\n')

Output

[{'Name': 'Alan Turing', 'age': 25, 'salary': 10000}, {'Name': 'John Hopkins', 'age': 18, 'salary': 1000}, {'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000}, {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000}]

[{'Name': 'John Hopkins', 'age': 18, 'salary': 1000}, {'Name': 'Alan Turing', 'age': 25, 'salary': 10000}, {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000}, {'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000}]

[{'Name': 'Mikhail Tal', 'age': 40, 'salary': 15000}, {'Name': 'Alan Turing', 'age': 25, 'salary': 10000}, {'Name': 'Sharon Lin', 'age': 30, 'salary': 8000}, {'Name': 'John Hopkins', 'age': 18, 'salary': 1000}]

Recommended Readings

• Python Lambda
• Python sorted()