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Posted: November 27th, 2021

List Comprehensions

What is a list comprehension and why use it?

A list comprehension in Python is a way of creating a new list from an iterable object by performing an operation over each of its items! While it can seem quite odd and cryptic at first, it comes with benefits such as using less lines of code, making things more compact! Further more, it is faster than using say a conventional for-loop, because Python will allocate the list memory first, before adding the elements to it, (as opposed to having to resize on runtime). When fully understood, these lists become quite readable too!

We'll start off by using a simple example without using a list comprehension. Say we want to get all the squared numbers from 0 to 10. A traditional loop could look something like this:

Example 1a (using a for loop)

square_numbers = [] # create an empty list
for i in range(11): # The last number within the range() method is not included
    square_numbers.append(i * i)
print(square_numbers)

Output: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
This works, but the code is rather bulky! We can recreate this using less code and thus simplifying things:

Example 1b (using a list comprehension instead)

square_numbers = [i * i for i in range(11)]
print(square_numbers)

Output: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
The output is the same, but when using a list comprehension, a new list is automatically created with new items only when the condition is met involving the existing one.



The basic formats of list comprehensions are as follows:

List Comprehension Syntaxes:

  • new_list = [expression for item in iterable object]
  • new_list = [expression for item in iterable object (if conditional)]
  • new_list = [expression for item in iterable object (if conditional_1) and (if conditional_2)]
  • new_list = [expression for item_1 in iterable object and item_2 in iterable object]

For clarity sake, it's important understand what each element of the syntaxes means:

  • expression: This can be an item itself, a method, mathematical operation, a conditional statement or even another list comprehension! Ultimately, it returns a value. In example 1b, i * i is the expression.
  • item: also known as a member, this is the object / value in the iterable object. In example 1b, the member value is i.
  • iterable object: is an object that can return its elements one at a time. This can include a list, set, sequence or generator. In example 1b, the iterable object is range(11).
  • conditional(s): optional - tests any valid expression. It be useful in filtering out unwanted values for example.


Let's examine a few more examples showcasing other various formats!

example 2 (using a conditional)

even_numbers = [i for i in range(15) if i%2 == 0]
print(even_numbers)

Output: [0, 2, 4, 6, 8, 10, 12, 14]
In the example above, we made use of a single conditional test. Whenever the test is successful during each iteration, the returned result is added to the new_list!

example 3a (using 'and' conditional)

names = ["Steph", "Joey", "David", "Sarah", "Otis", "Julie", "Daniel", "Alexis", "Stan"]
name_fetch = [i for i in names if i.startswith("S") and i.endswith("h")]
print(name_fetch)

Output: ['Steph', 'Sarah']
Here, we made use of two conditional tests. The code looks through the list of names and returns any names that start with "S" and ends with "h". The name Stan was not included in this, as it fails the test.



An alternative version of the code above can use two if conditionals back to back like so:

example 3b (using dual if conditionals)

names = ["Steph", "Joey", "David", "Sarah", "Otis", "Julie", "Daniel", "Alexis", "Stan"]
name_fetch = [i for i in names if i.startswith("J") if i.endswith("y")]
print(name_fetch)

Ouput: ['Joey']
Notice the lack of the 'and' logical operator keyword in line 2. You can optionally use 'and' or simply string two if conditionals consecutively. Either version will do the trick!

example 4 (using dual if conditionals)

numbers_list = [number for number in range(1, 11)]
squares = [n * n for n in numbers_list]
n_plus_square = [n + s for n, s in zip(numbers_list, squares)]
print(n_plus_square)

Output: [2, 6, 12, 20, 30, 42, 56, 72, 90, 110]
The first line uses a list comprehension to store the numbers 1 thru 10 into the list called numbers_list (remember, the last number in the range is not included, so to get to 10, we must state 11)! In turn, line 2 uses a list comprehension to iterate through the numbers_list by multiplying each item with itself to create a list of squares.

Finally, line 3 creates yet another list (n_plus_square) to make use of a list comprehension to add n + s (both represented as iterations of numbers_list and squares respectively), all being dealt with using a zip function (which basically involves parallel iterations by using iterables as arguments and returns an iterator that creates a series of tuples containing elements from each iterable listed within it)!

So the end result is the range of numbers (numbers_list) which a squares list multiplies them by themselves and both are used brought in and added to each other. Notice how compact these three lines are!

Conclusion


Once understood, list comprehensions makes code more compact, yet easy to understand as well as perform faster than alternative ways! ∎