Python 1 line for loop

Instead of using the same old way of iterating through lists, we can make our code simpler by using list comprehensions, which allow us to make a 1 line for loop in Python.

Basic syntax of a 1 line for loop

To use a one line for loop to replace a regular for loop, we can make:

[statement for i in list]

Which is the same as doing:

for i in list:
    expression

If we want some conditional to apply the expression, we have:

[statement for i in list if condition ]

Which translates to:

for i in list:
    if condition:
        statement

Example 1: calculating the cube of a number

Regular way

numbers = [10, 20, 30, 40, 50]
new_list = []

for n in numbers:
    new_list.append(n**3)

print(new_list)

[1000, 8000, 27000, 64000, 125000]

Using 1 line for loop

numbers = [10, 20, 30, 40, 50]
new_list = []

new_list = [n**3 for n in numbers]

print(new_list)

[1000, 8000, 27000, 64000, 125000]

Example 2: calculating the cube of a number only if it is greater than 30

Using the conditional, we can filter only the values we want the expression to be applied to.

Regular way

numbers = [10, 20, 30, 40, 50]
new_list = []

for n in numbers:
    if(n > 30):
        new_list.append(n**3)

print(new_list)

[64000, 125000]

Using 1 line for loop

numbers = [10, 20, 30, 40, 50]
new_list = []

new_list = [n**3 for n in numbers if n > 30]

print(new_list)

[64000, 125000]

Example 3: calling functions with a 1 line for loop

We can also call functions using the list comprehension syntax:

numbers = [10, 20, 30, 40, 50]
new_list = []

def cube(number):
    return number**3

new_list = [cube(n) for n in numbers if n > 30]

print(new_list)

[64000, 125000]

To know more about loops, check these posts on for Loops in Python and While Loops in Python.