Python one line for loop tutorial

To create a one line for loop in Python, you can use one of the following methods:

1. If the for loop body is simple, you can write the statement next to the colon
2. If you’re creating a list, use a list comprehension
3. If you have an if condition, use a conditional list comprehension

This tutorial shows you how to create one line for loops in practice.

1. One liner for loop

If you have a simple for loop, then you can write the loop body in the same line as the loop header:

for condition: do_something

Let’s see a practical example. Suppose you have a for loop as follows:

for i in range(5):
    print(i)

The code above can be converted to a single line for loop as follows:

for i in range(5): print(i)

If you have multiple statements, you can separate them into a single line using semicolons:

# This code:
for i in range(5):
    print("Start")
    print(i)
    print("End")

# Equivalent to:

for i in range(5): print("Start"); print(i); print("End")

But keep in mind that writing a single line for loop goes against Python conventions of one statement per line.

Only use this syntax if you have a strong reason to.

2. Using a list comprehension and for loop

The for loop can be combined with a list comprehension to create a list.

Suppose you want to add a +2 to a range of numbers from 1 to 5. Here’s how you do it using a regular for loop:

result = []

for i in range(1, 6):
    result.append(i + 2)
    
print(result)

# [3, 4, 5, 6, 7]

You can shorten the code above by using a list comprehension as follows:

result = [i + 2 for i in range(1, 6)]

print(result)

# [3, 4, 5, 6, 7]

Isn’t that simple? List comprehension allows you to create a list and do operations on the list items in one line. You also don’t have to declare a list variable before running the list comprehension.

Next, let’s see how to add an if statement in a list comprehension

3. Conditional list comprehension

If you want to create a list that fulfills a certain condition, you can add an if statement to the list comprehension.

For example, say you want to create a list of even numbers from a range. This is how you do it with a regular for loop:

even = []

for number in range(1, 11):
    if number % 2 == 0:
        even.append(number)

print(even)

# [2, 4, 6, 8, 10]

Now here’s how you do the same task using a conditional list comprehension:

even = [number for number in range(1, 11) if number % 2 == 0]

print(even)

# [2, 4, 6, 8, 10]

It’s a bit nauseating, but essentially the list comprehension uses a for loop to iterate over a range of numbers. Each number that returns True in the if check gets added to the list.

Aside from being hard to read, a conditional list comprehension also has limited support for else statements.

Suppose you want to create a new list that squares each even number in the range as follows:

squared = []

for number in range(1, 11):
    if number % 2 == 0:
        squared.append(number ** 2)
    else:
        squared.append(number)

print(squared)

# [1, 4, 3, 16, 5, 36, 7, 64, 9, 100]

You can create a one line conditional comprehension as follows:

numbers = range(1, 11)

squared = [num ** 2 if num % 2 == 0 else num for num in numbers]

print(squared)
#  [1, 4, 3, 16, 5, 36, 7, 64, 9, 100]

But suppose you want to create a two list out of the numbers. Let’s say you want to create two lists, one for even numbers and another for odd numbers:

odd = []
even = []

for number in range(1, 11):
    if number % 2 == 0:
        even.append(number)
    else:
        odd.append(number)

print(odd)
print(even)

# [1, 3, 5, 7, 9]
# [2, 4, 6, 8, 10]

The example above uses two lists, and it can’t be done using a conditional comprehension.

The best you can do is to use the ternary operator and put the if-else statement in one line as follows:

odd = []
even = []

for number in range(1, 11):
    even.append(number) if number % 2 == 0 else odd.append(number)

print(odd)
print(even)

# [1, 3, 5, 7, 9]
# [2, 4, 6, 8, 10]

You can even write the for loop body in one line like this:

for number in range(1, 11): even.append(number) if number % 2 == 0 else odd.append(number)

But of course, this is discouraged in practice as it’s hard to read. Don’t blame me if someone scolded you for it. 😬

Conclusion

This tutorial has shown you examples of writing a one line for loop in Python.

Writing a one line for loop goes against Python code conventions that state you must not have more than one statement per line. But if you have a really simple for loop condition in your project, then by all means go for it.

I hope this tutorial is helpful. Happy coding! 👍