Python List Comprehensions

List comprehensions in Python provide a powerful and concise way to create lists. They are an essential part of any Python programmer's toolkit. Ready to make your code more Pythonic? Let's dive in!

What is a List Comprehension?

A list comprehension is a compact way to process all or part of the elements in a sequence and return a list with the results. The syntax is simple yet powerful:

[expression for item in iterable if condition]

Basic Example

Let's start with a basic example. Suppose we want a list of squares for numbers from 0 to 9. With list comprehensions, it's a one-liner:

squares = [x**2 for x in range(10)]

print(squares)

Output:

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

Adding a Condition

What if we want only even squares? Just add an if condition at the end:

even_squares = [x**2 for x in range(10) if x % 2 == 0]

print(even_squares)

Output:

[0, 4, 16, 36, 64]

Nested Comprehensions

You can also nest comprehensions for multidimensional lists. Here’s an example to flatten a 2D list:

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

flattened = [num for row in matrix for num in row]

print(flattened)

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Dictionary Comprehensions

List comprehensions aren’t just for lists. You can create dictionaries too!

squares_dict = {x: x**2 for x in range(10)}

print(squares_dict)

Output:

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}

Set Comprehensions

And even sets! This creates a set of unique squares:

unique_squares = {x**2 for x in range(10)}

print(unique_squares)

Output:

{0, 1, 4, 9, 16, 25, 36, 49, 64, 81}

Advanced Use - Function Application

You can apply functions within comprehensions. Here’s an example using str.upper():

words = ["hello", "world", "python"]

upper_words = [word.upper() for word in words]

print(upper_words)

Output:

['HELLO', 'WORLD', 'PYTHON']

Comprehensions with Multiple Conditions

You can add multiple conditions. Let’s filter numbers divisible by 2 and 3:

filtered = [x for x in range(20) if x % 2 == 0 if x % 3 == 0]

print(filtered)

Output:

[0, 6, 12, 18]

In Conclusion

List comprehensions make your code cleaner and more readable. They’re efficient and Pythonic. Practice and integrate them into your projects to see the magic!