Posted on Nov 7, 2020

List comprehensions in Python

#python #functional #tutorial #beginners

In my previous post, I covered How to use map, filter and reduce in Python.

Here I demonstrate how to use a List comprehension to achieve similar results to using map and filter together.

You can use a list comprehension to iterate over an iterable, like a string, list or tuple and create new list,

Basic

The simplest kind does nothing.

a = [1, 2, 3] b = [x for x in a] b # [1, 2, 3]

Map style

Here we apply something like the map call.

a = [1, 2, 3] b = [x**2 for x in a] b # [1, 4, 9]

Note you don't need a lambda.

If you had a function, you would use it like this:

b = [square(x) for x in a] b # [1, 4, 9]

Using a string instead of a list.

c = 'aBcD' c.isupper() # False  [x for x in c if x.isupper()] # ['B', 'D']

Filter style

This is a way to an if statement inside the list comprehension to filter out values.

a = [1, 2, 3] [x for x in a if x > 2] # [3]  [x for x in a if x > 1] # [2, 3]  [x**2 for x in a if x > 1] # [4, 9]

Map and filter combined

You can use the map and filter styles together.

a = [1, 2, 3] [x**2 for x in a if x > 2] # [9]  [x**2 for x in a if x**2 > 2] # [4, 9]

Generator

As a bonus, you can use a list comprehension as a generator if you use round brackets instead of square brackets.

I have hardly need to use this, but if you need to optimize your performance or you are working with large external data in a file or a REST API, you could find this useful. As it reduces how much is processed and stored in memory at once.

Standard:

a = [1, 2, 3] b = [x**2 for x in a] [1, 4, 9]

Generator:

c = (x**2 for x in a) c # <generator object <genexpr> at 0x1019d0f90> list(c) [1, 4, 9]

Or you might use a for loop to iterate over your initial list.

Here we chain two generators together. Computation is done lazily - both generators are only evaluated at the end.

c = (x**2 for x in a) d = (x+1 for x in c) list(d) # [2, 5, 10]

That is similar to this, which applies both transformations at once.

d = (x**2 + 1 for x in a) list(d) # [2, 5, 10]

Or with function calls.

d = (add_one(square(x)) for x in a) list(d) # [2, 5, 10]

DEV Community