Python List Sorting

Sometimes we might need to sort lists in Python programming. This article will discuss Python list sorting methods.

Using list.sort method

Python lists have a built-in sort method it modifies the list in place.

The syntax of the list.sort method

sort(*, key=None, reverse=False)

Parameters

• key : Callable function that takes a single argument
• reverse : a boolean value to indicate comparison in reverse

Let’s see some examples of list.sort method.

numbers = [10,1,500,20,70,60,8,122]
numbers.sort()
print(numbers)

Output

[1, 8, 10, 20, 60, 70, 122, 500]

Reverse sorting by passing reverse=True to sort method. The list will be sorted in descending order.

numbers = [10,1,500,20,70,60,8,122]
numbers.sort(reverse=True)
print(numbers)

Output

[500, 122, 70, 60, 20, 10, 8, 1]

Using sorted function

The sorted built-in function can be used to sort lists.

The syntax of the sorted function

sorted(iterable, key=None, reverse=False)

The sorted function has two optional arguments that must be specified as keyword arguments.

Parameters

• key : Callable function that takes single argument
• reverse : a boolean value to indicate comparison in reverse

Returns

A new sorted list with the items in the iterable

Let’s see some examples of Python list sorting using the sorted function.

numbers = [10,1,500,20,70,60,8,122]
sorted_nums = sorted(numbers)
print(numbers)
print(f"Sorted list - {sorted_nums}")

Output

[10, 1, 500, 20, 70, 60, 8, 122]
Sorted list - [1, 8, 10, 20, 60, 70, 122, 500]]

Reverse sort order

numbers = [10,1,500,20,70,60,8,122]
sorted_nums = sorted(numbers,reverse = True)
print(numbers)
print(f"Sorted list - {sorted_nums}")

Output

[10, 1, 500, 20, 70, 60, 8, 122]
Sorted list - [500, 122, 70, 60, 20, 10, 8, 1

Key functions in sorted/list.sort()

The key parameter is a function or callable. The sort function will call the key function on each element before comparison.

message = "This is a test message from NoloWiz"
sorted_message = sorted(message.split(),key=str.lower)
print(sorted_message)

Output

['a', 'from', 'is', 'message', 'NoloWiz', 'test', 'This']

We have used str.lower as key function.

Sorting tuples

In this example, we will sort employees’ tuples based on their salary.

employees = [
    ('Tony', 'developer', 1000),
    ('Robert', 'tester', 800),
    ('Ram', 'manager', 4000),
]

employees_sorted = sorted(employees, key =lambda employee: employee[2]) #sort salary
print(employees_sorted)

Output

[('Robert', 'tester', 800), ('Tony', 'developer', 1000), ('Ram', 'manager', 4000)]

Sort class objects

In this example, we will create a list of objects and sort.

class Employee:
    def __init__(self, name, desig, salary):
        self.name = name
        self.designation = desig
        self.salary = salary

    def __repr__(self):
        return repr((self.name, self.designation, self.salary))

employee_objects = [
    Employee('Tony', 'developer', 1000),
    Employee('Robert', 'tester', 800),
    Employee('Ram', 'manager', 4000),
]

employees_sorted = sorted(employee_objects, key=lambda employee: employee.salary)   # sort by salary
print(employees_sorted)

Output

[('Robert', 'tester', 800), ('Tony', 'developer', 1000), ('Ram', 'manager', 4000)]

Using itemgetter itemgetter(), attrgetter() with sorting

The operator module has itemgetter(), attrgetter() funtions,so instead of using lambda expression we can use itemgetter/attrgetter functions as key function.

from operator import itemgetter, attrgetter

employees_sorted = sorted(employees, key=itemgetter(2))
print(employees_sorted)

Output

[('Robert', 'tester', 800), ('Tony', 'developer', 1000), ('Ram', 'manager', 4000)]

Let’s see an example of object sorting using attrgetter.

from operator import itemgetter, attrgetter

employees_sorted = sorted(employee_objects, key=attrgetter("salary"))
print(employees_sorted)

Output

[('Robert', 'tester', 800), ('Tony', 'developer', 1000), ('Ram', 'manager', 4000)]

Here attrgetter function used to get employee object’s attribute “salary”.

Conclusion

These are the different ways to sort lists in Python. Read remove list duplicates.