Python List Comprehension (With Examples)

Pre-requisites

1. Python Lists
2. Sequence vs Iterable vs Iterator
3. next() in Python
4. for loop - Behind the scenes
5. A file object is an iterator
6. Unpacking an iterable in Python
   

List Comprehensions

Along with for loops, list comprehensions are one of the most useful contexts in which the iteration protocol is applied.

List comprehensions, allow you to write code that is shorter and more efficient.

Example 1

Code

# using for loop
my_list = [10, 20, 30, 40]
for i in range(len(my_list)):
    my_list[i] *= 2
print(my_list)

# using list comprehension
my_list = [10, 20, 30, 40]
new_list = [x*2 for x in my_list]
print(new_list)

Output

[20, 40, 60, 80]
[20, 40, 60, 80]

As you can see, you are able to replace the loop with a single expression that produces the desired result list.

Note:- List comprehensions aren’t exactly same as the for loop statement version because it makes a new list object.

For loops perform computation on a list object, list comprehension creates a new one.

Example 2

Code

# using for loop
squares_list = []
for i in range(10):
    squares_list.append(i**2)
print(squares_list)

# using list comprehensions
squares_list = []
new_squares_list = [i**2 for i in range(10)]
print(new_squares_list)

Output

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Understanding list comprehensions syntax

Usual syntax of a list comprehension is

[expression for item in iterable]

Let’s dissect the previous example to understand the syntax of list comprehensions.

Code

squares_list = []
new_squares_list = [i**2 for i in range(10)]
print(new_squares_list)

As you can see, list comprehensions are written in square brackets as they are used to create a new list. They begin with an expression which uses a loop variable i**2(i is the loop variable and i**2 is the expression). This is followed by the header of a for loop, which names the loop variable and an iterable object for i in range(10).

If you notice carefully, the syntax of list comprehension looks like a backwards for loop.

Using conditions in List comprehensions

Similar to using if-elif-else conditions in for loops, list comprehensions can also use conditionals.

Example 3

Let’s write a Python program to add even numbers to a list from range(10).

Code

# using for loops
my_list = []
for i in range(10):
    if i%2 ==0:
        my_list.append(i)
print(my_list)

#using list comprehensions
my_list = [i for i in range(10) if i%2==0]
print(my_list)

Output

[0, 2, 4, 6, 8]
[0, 2, 4, 6, 8]

Example 4 - if else in a list comprehension

Notice how the if clause in the previous example comes after the iterable(range(10)). If you want to include both if else conditions in list comprehensions, you have to move the conditionals before the iterable.

Let’s look at this below example.

Code

# using for loop
my_list = [2, 5, 8, 11, 15]
for i in range(len(my_list)):
    if my_list[i] < 10:
        my_list[i] *=2
    else:
        my_list[i] += 1
print(my_list)

# using list comprehension
my_list = [2, 5, 8, 11, 15]
new_list = [x*2 if x < 10 else x+1 for x in my_list]
print(new_list)

Output

[4, 10, 16, 12, 16]
[4, 10, 16, 12, 16]

Notice how if x < 10 else x+1 comes before for x in my_list.

Example 5 - Nested if with List comprehension

Let’s write a Python program to append numbers that are divisible by both 2 and 5 between 2 sets of integers.

Code

# using for loops
my_list = []
for i in range(1, 30):
    if i%2 ==0:
        if i%5==0:
           my_list.append(i)
print(my_list)

# using list comprehension
my_list = [i for i in range(1, 30) if i%2==0 if i%5==0]
print(my_list)

Output

[10, 20]
[10, 20]

Here list comprehension checks 1. if i is divisible by 2, and then, 2. if i is divisible by 5.

If i is divisible by both, it is appended to the my_list variable.

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