Learn Python List Comprehension

Hello Pythonista, Today you’re going to learn about python comprehension, a very powerful feature to use when creating a python List based on certain constraints.

let’s get started

Imagine you want to create a list of cubic numbers from 1 to 100, Generating the cubic of numbers without using list comprehension would normally look like this.

Cubic of numbers without list comprehension

>>> cubics = []
>>> for number in range(1, 101):
...     cubic = number**3
...     cubics.append(cubic)
... 
>>> cubics
[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728, 
 2197, 2744, 3375, 4096, 4913, 5832, 6859, 8000, 9261, 10648,
 ......
]

Cubic of numbers using list comprehension

>>> cubics = [number**3 for number in range(1, 101)]
>>> print(cubics)
[
  1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728, 2197, 2744,
 3375, 4096, 4913, 5832, 6859, 8000, 9261, 10648, 12167, 13824, 15625,
 ....
]

As we can see, we did the same thing with list comprehension but instead of doing it in 4 lines of code, we did it in just one. is it cool?

If you’re a fan of one-liners then list comprehension should be your friends, it can do amazing stuff plus the fact you can also put conditions blocks in it.

List comprehension Syntax

List = [element for element in iterable]

Cartesian Product using List Comprehension

If A and B are two non-empty sets, then their Cartesian product A × B is the set of all ordered pairs of elements from A and B, with list comprehension you find it with one line as shown below;

>>> a = [1 , 3 , 5, 7]
>>> b = [2, 4, 6, 8]
>>> product = [(i , j) for i in a for j in b]
>>> print(product)
[(1, 2), (1, 4), (1, 6), (1, 8), (3, 2),
 (3, 4), (3, 6), (3, 8), (5, 2), (5, 4),
 (5, 6), (5, 8), (7, 2), (7, 4), (7, 6), 
 (7, 8)
]

Conditional blocks within a list comprehension

Apart from just iterating over sequences, you can also put conditional statements such as if and else within comprehension logic to select the elements based on certain conditions.

Even number using list comprehension

Let’s say we want to make a list of even numbers from 203 to 289 using list comprehension, it can also be done in one line as shown below;

>>> even_numbers = [number for number in range(203, 289) if number%2==0]
>>> print(even_numbers)
[
  204, 206, 208, 210, 212, 214, 216, 218, 220, 222, 224, 226, 228, 230,
 232, 234, 236, 238, 240, 242, 244, 246, 248, 250, 252, 254, 256, 258,
 260, 262, 264, 266, 268, 270, 272, 274, 276, 278, 280, 282, 284, 286,
 288
]

Filtering iterable using list comprehension

This an example program that uses a list comprehension to filtering candidates that are above or equal to 18.

>>> candidates = [
...     ('John', 15), 
...     ('Silyvia', 34), 
...     ('Hamis', 17), 
...     ('Alphonce', 22), 
...     ('Grace', 27)
... ]
>>> 
>>> above_18 = [candidate for candidate in candidates if candidate[1]>=18]
>>> print(above_18)
[('Silyvia', 34), ('Alphonce', 22), ('Grace', 27)]Loading certain files using list comprehension

Let’s use list comprehension to load all image paths in our current directory

>>> import os
>>> images = [img for img in os.listdir() if img.endswith('.jpg')]
>>> print(images)
['AI gallery.jpg']

Flattening 2D array using list comprehension

You can also use list comprehension to flatten the 2D array to a single 1D array by iterating on all elements on a list.

>>> data =[(1, 2), (3, 4), (5, 6)]
>>> flattened = [n for in_data in data for n in in_data]
>>> print(flattened)
[1, 2, 3, 4, 5, 6]

Scalar Product using List Comprehension

List comprehension can also be used to find the scalar of an iterable by multiplying by a given number on each element in an iterable independently as shown below;

>>> vector = [10, 25, 32]
>>> vector3 = [n * 3 for n in vector]
>>> print(vector)
[10, 25, 32]

Based on your interest I recommend you also check;

• how to make a digital clock in python
• Basics of python OS Module
• File handling in Python
• When Decimal is better than float (Python)
• Interesting python tricks you should know

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Originally published at https://kalebujordan.com on June 7, 2020.