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Python List Comprehension: One-Liner For Loops

Comprehensions offer a smooth approach for creating new sequences in a concise and readable way.

Python List Comprehension makes it possible to write concise one-liners for loops.

For instance, you can use a comprehension to replace this:

With this:

There are actually 4 different comprehensions in Python for the main collection types:

• List Comprehensions
• Dictionary Comprehensions
• Set Comprehensions
• Generator Comprehensions

Python List Comprehension

Here is the general structure for list comprehension for your convenience:

[expression for var in input_list if condition]

Where the if condition part is optional

Example

Say you have a list of numbers and you want to exclude all the negatives. Using a for loop to iterate over the list of numbers is nothing new:

Output:

[1, 3, 5]

However, if you want to bring it to the next level, you can make this whole for loop shorter by using list comprehension:

Output:

[1, 3, 5]

Dictionary Comprehensions

It is no surprise that Python has a similar shorthand for looping through dictionaries, called dictionary comprehension. The syntax for dictionary comprehension looks like this:

{ key:value for (key,value) in dict if key,value satisfy condition }

Where if key,value satisfy condition is optional.

Example 1 — Create a Dictionary from a List

Say you want to create a dictionary based on a numbers list. In this new dictionary, a number is a key and the value is the number as a string. Furthermore, you only want to include even numbers:

Output:

{2: '2', 4: '4', 6: '6', 8: '8'}

This works all fine but by using dictionary comprehension, this all can be achieved with one line!

Output:

{2: '2', 4: '4', 6: '6', 8: '8'}

Example 2— Operating on an Existing Dictionary

Let’s see another example where you already have a dictionary and want to create a new one based on it. This could be done with a regular for loop again, but let’s directly use a dictionary comprehension:

Output:

{'a': 3, 'b': 6, 'c': 9, 'd': 12, 'e': 15}

Set Comprehensions

Set comprehension is like list comprehension that works for sets. Here is the general structure for set comprehension:

{ expression for var in input_list if condition }

Where the if condition part is optional

Example

Let’s see an example where a list of numbers is transformed into a set leaving all the odd numbers out. This can be done with a multi-line loop approach again:

Output:

{8, 2, 4, 6}

But with set comprehension this can be achieved with one line:

filtered_nums = { num for num in numbers if num % 2 == 0 }
print(filtered_nums)

Generator Comprehensions

Generator comprehension is also similar to list comprehension but works with generators. Here is the general structure for generator comprehension:

( expression for var in input_list if condition )

For example, let’s square all the even numbers in a numbers list and leave out all the odd numbers using a generator:

Output:

4 16 36

With generator comprehension, you can forget about the square_even method and get what you want simply by:

Output:

4 16 36

Conclusion

Python list comprehension is a concise way of creating new lists with one-liner expressions. It can improve code quality when used smartly.

Python List comprehensions are not the only comprehensions out there. In total, there are 4 types of comprehensions:

• List Comprehension
• Dictionary Comprehension
• Set Comprehension
• Generator Comprehension

Thanks for reading. Happy coding!

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