How to Use List Comprehensions in Python

March 2, 2024
Posted by Farazin3D

What are list comprehensions?

List comprehensions are expressions that create a new list from an existing iterable, such as a list, a tuple, a set, a dictionary, or a range. The syntax of a list comprehension is:

[expression for item in iterable if condition]

The expression can be any valid Python expression that uses the item variable. The iterable can be any object that supports iteration. The optional condition can be used to filter the items that are included in the new list.

For example, suppose you have a list of numbers and you want to create a new list that contains only the even numbers from the original list. You can use a list comprehension like this:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = [n for n in numbers if n % 2 == 0]
print(even_numbers)
# [2, 4, 6, 8, 10]

The expression n simply returns the item itself, the iterable is the numbers list, and the condition is n % 2 == 0, which checks if the item is divisible by 2. The result is a new list that contains only the even numbers from the original list.

How to use list comprehensions?

List comprehensions can be used to perform various operations on iterables, such as mapping, filtering, flattening, or nesting. Here are some examples of how to use list comprehensions for different purposes.

Mapping

Mapping is the process of applying a function or a transformation to each item in an iterable and creating a new list with the results. For example, suppose you have a list of names and you want to create a new list that contains the lengths of each name. You can use a list comprehension like this:

names = ["Alice", "Bob", "Charlie", "David", "Eve"]
lengths = [len(name) for name in names]
print(lengths)
# [5, 3, 7, 5, 3]

The expression len(name) returns the length of the item, and the iterable is the names list. The result is a new list that contains the lengths of each name.

Filtering

Filtering is the process of selecting only the items in an iterable that satisfy a certain condition and creating a new list with them. For example, suppose you have a list of words and you want to create a new list that contains only the words that start with the letter “a”. You can use a list comprehension like this:

words = ["apple", "banana", "cherry", "date", "elderberry"]
a_words = [word for word in words if word.startswith("a")]
print(a_words)
# ["apple"]

The expression word simply returns the item itself, the iterable is the words list, and the condition is word.startswith("a"), which checks if the item starts with the letter “a”. The result is a new list that contains only the words that start with the letter “a”.

Flattening

Flattening is the process of converting a nested iterable, such as a list of lists, into a single list that contains all the items from the sublists. For example, suppose you have a list of lists that represents a matrix and you want to create a new list that contains all the elements from the matrix. You can use a list comprehension like this:

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
elements = [x for row in matrix for x in row]
print(elements)
# [1, 2, 3, 4, 5, 6, 7, 8, 9]

The expression x simply returns the item itself, the iterable is the matrix list, and there is no condition. However, notice that there are two for clauses in the list comprehension. The first one iterates over the sublists in the matrix, and the second one iterates over the items in each sublist. The result is a new list that contains all the elements from the matrix.

Nesting

Nesting is the process of creating a nested iterable, such as a list of lists, from a single iterable. For example, suppose you have a list of numbers and you want to create a new list that contains the squares and cubes of each number as sublists. You can use a list comprehension like this:

numbers = [1, 2, 3, 4, 5]
powers = [[n**2, n**3] for n in numbers]
print(powers)
# [[1, 1], [4, 8], [9, 27], [16, 64], [25, 125]]

The expression [n**2, n**3] returns a sublist that contains the square and the cube of the item, and the iterable is the numbers list. The result is a new list that contains the sublists with the powers of each number.

Tips and tricks for list comprehensions

List comprehensions are a great tool to write concise and elegant code, but they can also become complex and hard to read if not used properly. Here are some tips and tricks to make the most of list comprehensions and avoid common pitfalls.

Use descriptive variable names for the items in the iterable, such as name for a list of names, or word for a list of words. Avoid using generic names like x or i, unless they are very short and simple expressions.
Keep the expressions and conditions simple and clear. If they are too long or complicated, consider using a helper function or a regular for loop instead.
Avoid nesting too many list comprehensions, as they can become confusing and hard to follow. A good rule of thumb is to limit the nesting to two levels at most.
Use parentheses to group the list comprehension and make it more readable. This is especially useful if the list comprehension is part of a larger expression, such as a function call or an assignment.
Use comments to explain the logic and purpose of the list comprehension, especially if it is not obvious or intuitive.

Conclusion

List comprehensions are a powerful and concise way to create lists from other iterables in Python. They can make your code more readable and elegant, and also save you some lines of code. You can use list comprehensions to perform various operations on iterables, such as mapping, filtering, flattening, or nesting. However, you should also be careful not to overuse or abuse list comprehensions, as they can become complex and hard to read if not used properly. Follow the tips and tricks in this article to make the most of list comprehensions and avoid common pitfalls.