Search⌘ K
Home
Courses
The Practical Guide to Python for Scientists and Engineers

List Comprehensions

Learn about list comprehensions in Python.

We'll cover the following...

List comprehension: an old method

An essential part of Python is list comprehension. This will come up repeatedly in examples seen online, so we’re learning about them separately. List comprehension is technically not a part of NumPy, but we’ll introduce it here for a supplementary explanation of the lesson.

The following was the old standard for list comprehensions. Let’s look at the code and then we’ll discuss it in detail.

Python 3.8
#! /usr/bin/python
import numpy as np
def main():
    x = [5,10,15,20,25]
    # declare y as an empty list
    y = []
    # The not so good way
    for counter in x:
        y.append(counter / 5)
    print("\nOld fashioned way: x = {} y = {} \n".format(x, y))
    # The Pythonic way
    # Using list comprehensions
    z = [n/5 for n in x]
    print("List Comprehensions: x = {} z = {} \n".format(x, z))
    # Finally, numpy
    try:
        a = x / 5
    except:
        print("No, you can't do that with regular Python lists\n")
    a = np.array(x)
    b = a / 5
    print("With Numpy: a = {} b = {} \n".format(a, b))
    return "With Numpy: a = {} b = {} \n".format(a, b)
if __name__ == "__main__":
    main()

We’re importing numpy in line 3 as np (this means we don’t have to type numpy every time, which is a typical Python pattern).

Next, we’ll initialize x with five values and y as an empty list in lines 6–8 in the above code ...