Sorting with Lambda Functions in Python

In Python, we can use a lambda expression with the sorted() or sort() function to sort with a function. To use lambda with sorted(), pass the lambda expression to the ‘key’ argument.

lst = ["this","is","another","list","of","strings"]

list_sorted_by_second_letter = sorted(lst,key=lambda x: x[1])

print(list_sorted_by_second_letter) 

#output:
['of', 'this', 'list', 'another', 'is', 'strings']

You can do the same for the sort() function, which sorts a list in place.

lst = ["this","is","another","list","of","strings"]

lst.sort(key=lambda x: x[1])

print(lst) 

#output:
['of', 'this', 'list', 'another', 'is', 'strings']

In Python, lambda expressions are very useful for creating anonymous functions which can be applied on variables or collections of objects.

We can use lambda expressions when sorting a list using the sorted() or sort() functions. The lambda expression will tell the sorting algorithm which values to use to sort the list.

For example, we can sort a list of strings by the second character using a lambda expression.

To use lambda with sorted() or sort(), pass the lambda expression to the ‘key’ argument.

Below is a simple example of how to sort a list of strings by the second character in Python using a lambda expression.

lst = ["this","is","another","list","of","strings"]

list_sorted_by_second_letter = sorted(lst, key=lambda x: x[1])

print(list_sorted_by_second_letter) 

#output:
['of', 'this', 'list', 'another', 'is', 'strings']

Another example is sorting a list of strings which are made up of integers.

To sort the list of strings made up of integers, we can pass “key=int” to the sort() function. The following code will sort a list of strings made up of integers:

l = ["100","12","34","1","5","56","78"]

l.sort(key=int)

print(l) 

#output:
["1","5","12","34","56","78","100"]

Sorting a List of Objects with lambda in Python

Another example where the use of a lambda makes sense is when sorting a list of objects.

For example, you can sort a list of tuples by the second element easily with a lambda.

Below is how to sort a list of tuples by the second element in Python with the sorted() function and a lambda expression.

list_of_tuples = [('apple', 3), ('pear', 5), ('banana', 1), ('lime', 4)]

sorted_list = sorted(list_of_tuples, key=lambda t: t[1])

print(sorted_list)

#Output:
[('banana', 1), ('apple', 3), ('lime', 4), ('pear', 5)]

Sorting by Two Keys with lambda in Python

One such situation where we need to do a little more work to get the result we want is if we want to sort our data by two keys.

To sort a list of objects by two keys in Python, the easiest way is with the key parameter and a tuple of the keys you want to sort by.

Just pass the keys you want to sort by as a tuple for your sorting lambda expression.

Below is a simple example showing you how to sort a list of dictionaries by two keys in Python.

list_of_dicts = [{"name":"Bob","weight":100,"height":50},
{"name":"Sally","weight":120,"height":70},
{"name":"Jim","weight":120,"height":60},
{"name":"Larry","weight":150,"height":60}]

list_of_dicts.sort(key= lambda x: (x["weight"],x["name"]))

print(list_of_dicts)

#Output:
[{'name': 'Bob', 'weight': 100, 'height': 50}, {'name': 'Jim', 'weight': 120, 'height': 60}, {'name': 'Sally', 'weight': 120, 'height': 70}, {'name': 'Larry', 'weight': 150, 'height': 60}

Hopefully this article has been useful for you to learn how to sort with a lambda expression in Python.