List comprehension

List comprehension in Python provides a concise way to create lists.

It consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses.

The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it.

[expression for item in iterable if condition]

Create a list of squares for numbers from 0 to 9.

squares = [x**2 for x in range(10)]
print(squares)
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Create a list of even numbers from 0 to 9.

evens = [x for x in range(10) if x % 2 == 0]
print(evens)
# [0, 2, 4, 6, 8]

Flatten a 2D list into a 1D list.

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

Point to note here, the first row loop get executed, the value of row pass to next loop and get accessed, which finally retun the num.

Apply a function to each element in a list.

def square(x):
    return x * x

numbers = [1, 2, 3, 4, 5]
squares = [square(x) for x in numbers]
print(squares)
# [1, 4, 9, 16, 25]

Create a list of numbers from 0 to 99 that are divisible by both 3 and 5.

div_by_3_and_5 = [x for x in range(100) if x % 3 == 0 and x % 5 == 0]
print(div_by_3_and_5)
# [0, 15, 30, 45, 60, 75, 90]

List comprehensions can be powerful tools for creating and manipulating lists in a concise, readable manner.

They can replace loops and map/filter functions with a single line of code.