Python List sort() Function

The List sort() method sorts the elements of the list in place.

note

By default, sort() method arranges the elements in ascending order, but it can also sort them in descending order or according to a custom sorting function, by specifying function's parameters .

danger

Lists can only be sorted if their elements are comparable to each other!

Otherwise, a TypeError exception is raised.

Syntax

my_list.sort(key, reverse)

sort() Parameters

Python List sort() method parameters::

Parameter	Condition	Description
key	Optional	A function to specify the sorting criteria. Default value is None.
reverse	Optional	Setting it to True sorts the list in reverse order. Default value is False.

warning

If used, both the arguments must be specified as keyword arguments.

For example:

my_list.sort(key=len) # sort using len function
my_list.sort(reverse=True) # sort in reverse order
my_list.sort(key=len, reverse=True) # sort using len function and in reverse order

note

A function to be used as key must take a single value and return single value.

sort() Return Value

Python List sort() function does not return any value: it modifies the original list.

Examples

Example 1: Sort a List

The sort() method sorts the list of strings alphabetically (according to ASCII):

names = ['Tom', 'Anna', 'David', 'Ryan']
names.sort()
print(names) # Output: ['Anna', 'David', 'Ryan', 'Tom']

output

['Anna', 'David', 'Ryan', 'Tom']

and the list of numbers numerically:

numbers = [5, 6, 7, 1, 2, 3, 9, 8, 4, 0]
numbers.sort()
print(numbers) # Output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

output

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Example 2: Sort a List in Descending Order

You can sort a list in descending order by setting reverse parameter to True.

numbers = [5, 6, 7, 1, 2, 3, 9, 8, 4, 0]
numbers.sort(reverse=True)
print(numbers) # Output: [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

output

[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Example 3: Sort a List of Not Comparable Elements

You can not sort lists that have both numbers and strings in them, because Python does not know how to compare these values.

For example, this will raise a TypeError:

names = ['Tom', 3, 'Anna', 1, 2, 'David', 'Ryan']
names.sort() # raises TypeError

output

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    names.sort() # raises
TypeError: '<' not supported between instances of 'int' and 'str'

Example 4: Sort a List with Key parameters

The key parameter is used for more complex custom sorting: the key parameter allows to specify a function to be executed on each list item before making the comparisons.

For example, with a list of strings, specifying key=len (where len is the len() built-in function) sorts the strings by length, from shortest to longest:

names = ['Tom', 'Anna', 'David', 'Ryan']
names.sort(key=len)
print(names) # Output: ['Tom', 'Anna', 'Ryan', 'David']

output

['Tom', 'Anna', 'Ryan', 'David']

::: note

Note that, in this example, the comparison is done only using the length and not the ASCII order of the characters! So, strings with same length appear in the result in the same order of the original list.

For example, Anna and Ryan have same length but their position in the original list is different:

names1 = ['Tom', 'Anna', 'David', 'Ryan']
names2 = ['Tom', 'Ryan', 'David', 'Anna']
names1.sort(key=len)
names2.sort(key=len)

print(names1) # Output: ['Tom', 'Anna', 'Ryan', 'David']
print(names2) # Output: ['Tom', 'Ryan', 'Anna', 'David']

output

['Tom', 'Anna', 'Ryan', 'David']

:::

Example 5: Sort with Custom Function

You can also pass to the sort() function your own custom function as key function. A key function should take a single argument and return a key to use for sorting.

For example, sort a list of students (as list of tuple) by age (second element of the tuple):

def sortByAge(e):
  return e[1]	# return age

students = [('Tom', 30),('Anna', 23), ('David', 25)]
students.sort(key=sortByAge)
print(students) # Output: [('Anna', 23), ('David', 25), ('Tom', 30)]

output

[('Anna', 23), ('David', 25), ('Tom', 30)]

For example, sort a list of dictionaries by age (where age is key in each dictionary with a corresponding value)

def sortByAge(e):
  return e['age']	# return age

students = [{'name': 'Tom', 'age': 30},
            {'name': 'Anna', 'age': 23},
            {'name': 'David', 'age': 25}]
students.sort(key=sortByAge)
print(students) # Output: [{'name': 'Anna', 'age': 23}, {'name': 'David', 'age': 25}, {'name': 'Tom', 'age': 30}]

output

[{'name': 'Anna', 'age': 23}, {'name': 'David', 'age': 25}, {'name': 'Tom', 'age': 30}]

Example 6: Sort List with a Lambda Expression

A key function may also be created with the lambda expression. It allows us to in-line function definition.

For example, sort a list of students (as list of tuple) by age (second element of the tuple):

students = [('Tom', 30),('Anna', 23), ('David', 25)]
students.sort(key=lambda student: student[1])
print(students) # Output: [('Anna', 23), ('David', 25), ('Tom', 30)]

output

[('Anna', 23), ('David', 25), ('Tom', 30)]

Example 7: Sort List of String without Case-Sensitivity

By default, the sort() method sorts the list in ASCII order rather than actual alphabetical order: this means uppercase letters come before lowercase letters.

names = ['Tom', 'Anna', 'david']
names.sort()
print(names) # Output: ['Anna', 'Tom', 'david']

output

['Anna', 'Tom', 'david']

If you want to sort the values in regular alphabetical order, set key to str.lower: this allows the sort() function to treat all the list items as if they were lowercase without actually changing the values in the list.

names = ['Tom', 'Anna', 'david']
names.sort(key=str.lower)
print(names) # Output: ['Anna', 'david', 'Tom']

output

['Anna', 'david', 'Tom']

sort() method vs sorted() built-in function

The sort() method does not return anything, it modifies the original list. If you do not want to modify the original list, use sorted() built-in function because it returns a sorted copy of the list.

For example, get a sorted copy of the list with sorted() built-in function:

names = ['Tom', 'Anna', 'David']
sorted_names = sorted(names)
print(names) # Output: ['Tom', 'Anna', 'David']

output

['Tom', 'Anna', 'David']

For example, iterate through a sorted list without changing the original

names = ['Tom', 'Anna', 'David']
for name in sorted(names):
    print(name)

output

Anna
David
Tom

Another difference is that:

• the sort() method is only defined for lists.
• the sorted() function accepts ANY iterable like tuple, dictionary etc.

Moreover, the sort() method doesn’t need to create a new list, so it’s faster between the two.