Sorting a list

After you have learned how to create and perform basic operations with lists, it is time to explore how to put their items in the order you need. There are two similar ways to do this: the method list.sort() and the function sorted(list). They have much in common and take the same arguments, so let's see how they work.

The sorting

Invoking list.sort() as well as sorted(list) sorts the list in the ascending order according to the natural order of stored elements.

# Invoking list.sort()
numbers = [3, 2, 5, 4, 1]
numbers.sort()
print(numbers)  # [1, 2, 3, 4, 5]

# Invoking sorted(list)
numbers = [3, 2, 5, 4, 1]
print(sorted(numbers))  # [1, 2, 3, 4, 5]

That is why an attempt to use print(numbers.sort()) will lead to the None result while the same attempt with sorted(list) gives you the result you need. This is the important distinction between sorted(list) and list.sort().

print(numbers.sort()) # None

The sorted function has one more feature: it works not only with lists but also with other collections, so you can also sort sets, dictionaries, tuples, and so on. The list.sort() method, on the contrary, works only with lists.

To sort a list in the descending order, you need to specify the reverse argument as True (it's False by default).

numbers = [3, 2, 5, 4, 1]
print(sorted(numbers, reverse=True))  # [5, 4, 3, 2, 1]

# the same with list.sort()
numbers.sort(reverse=True)
print(numbers)  # [5, 4, 3, 2, 1]

Both sorted(list) and list.sort() can sort strings according to their lexicographic order (as in a dictionary).

strings = ['aa', 'b', 'aaa', 'q', 'qq']
strings.sort()
print(strings)  # ['aa', 'aaa', 'b', 'q', 'qq']

However, if your list has uncomparable types (like strings and integers together), an error will occur both in list.sort() and in sorted(list):

TypeError: '<' not supported between instances of 'str' and 'int'

The key

There is one more argument that can be used with both functions. You can pass the key function, this function will be applied to each element in the list, and the list will be sorted depending on the results. For example, you can use key=len to sort words by their length.

names = ['Mary', 'James', 'Tom', 'Katarina', 'John']
names.sort(key=len)
print(names)  # ['Tom', 'Mary', 'John', 'James', 'Katarina']

You can also use lambda functions as a key. For example, if you need to sort numbers by the remainder of dividing by two, you can use a lambda function and the % operator. In the example below, 4 is moved to the first place (the remainder is 0), followed by the three other digits with the remainder 1. Note that the relative order of 7, 1, and 5 in the sorted list remains as it was in the initial one.

nums = [7, 4, 1, 5]
print(sorted(nums, key=lambda x: x % 2))  # [4, 7, 1, 5]

What is more, you can use custom functions for the value of the key argument. Let's say you have a list of float numbers and you want to sort these numbers by their fractional part. To do so, you can create a function that takes a number as its input and returns the fractional part of this number. The function below performs it by subtracting an integer part from the given number.

def my_sorted(x):
    return x - int(x)

numbers = [1.5, 3.2, 4.3]
print(sorted(numbers, key=my_sorted))  # [3.2, 4.3, 1.5]

The reverse

There is one more way to modify the order of elements in a list; you can reverse them. It can be done with the help of either thelist.reverse() method or by using the reversed() function.

Invoking list.reverse() reverses the order of elements in the list. Just as list.sort(), it operates in place and returns None, so you can't assign the result to another variable, instead, the initial list will be changed.

initial_list = [1, 2, 3, 4, 5]
reversed_list = initial_list.reverse()
print(reversed_list)  # None
print(initial_list)   # [5, 4, 3, 2, 1]

The function reversed() returns a reverse iterator, by passing which, you can get the elements of the input sequence in the reverse order. To get access to the reversed list, you can write the following:

numbers = [1, 2, 3, 4, 5]
for number in reversed(numbers):
    print(number) 
# 5 
# 4
# 3
# 2
# 1

Pay attention that if you try to print the iterator without using a loop, you will not get the needed result:

print(reversed(numbers))  # <list_reverseiterator object at 0x7fe25e718b70>

Summary

In this topic, we have learned how to sort lists using the function sorted() and the method list.sort(); we've also found out the difference between them and their arguments. Besides, we have learned how to reverse elements in a list with the help of both the function and the method.

To sum up:

• list.sort() and list.reverse() work only with list objects; they change the given list and return nothing;
• sorted() takes any collection as an argument and creates and returns a new item from the given one;
• reversed() can also take any collection as an argument but returns the reversed iterator of the object rather than the modified object itself;
• list.sort() and sorted() can take the boolean parameter reverse to change the order of sorting from ascending to descending, as well as the key parameter to specify the sorting function.

For additional information about sorting methods, you can check out the Official Python documentation.