Python Convert String List to Lowercase

Coding Challenge 🥋

💬 Question: How to convert a list of strings to all lowercase in Python?

Here are three examples:

• ['HELLO', 'Python', 'world'] to ['hello', 'python', 'world']
• ['A', 'B', 'C'] to ['a', 'b', 'c']
• ['AA AA', 'BB$BB', 'CC()CC'] to ['aa aa', 'bb$bb', 'cc()cc']

There are multiple great ways to accomplish this coding challenge and I strongly believe it’s a great learning tool to dive into each of them. So, let’s get started with my most recommended solution first! 👇

Method 1: List Comprehension + str.lower() 🐍

The most Pythonic way to convert a string list my_list to all lowercase strings is to use the str.lower() method inside a list comprehension expression like so: [x.lower() for x in my_list]. The result is a new string list, all lowercase.

Here’s this method applied to our example strings:

def all_lower(my_list):
    return [x.lower() for x in my_list]


print(all_lower(['HELLO', 'Python', 'world']))
# ['hello', 'python', 'world']

print(all_lower(['A', 'B', 'C']))
# ['a', 'b', 'c']

print(all_lower(['AA AA', 'BB$BB', 'CC()CC']))
# ['aa aa', 'bb$bb', 'cc()cc']

If you struggle with list comprehension, I’d highly urge you to watch the following explainer video and dive into the related tutorial:

A Simple Introduction to List Comprehension in Python

🌍 Recommended Tutorial: List Comprehension in Python — A Helpfull Illustrated Guide

If you want to convert a string list to all uppercase, check out this detailed tutorial on the Finxter blog.

Method 2: Map Function + String Lower 👌

A concise Python one-liner to convert a string list to all lowercase strings is to use the map() function with a lambda function as a first argument — converting a string argument x to its lowercase variant using x.lower() — and the list of strings as the second argument. The result is a map object that you can convert to a list using the list() built-in function.

Here’s an example on the same sample lists:

def all_lower(my_list):
    return list(map(lambda x: x.lower(), my_list))


print(all_lower(['HELLO', 'Python', 'world']))
# ['hello', 'python', 'world']

print(all_lower(['A', 'B', 'C']))
# ['a', 'b', 'c']

print(all_lower(['AA AA', 'BB$BB', 'CC()CC']))
# ['aa aa', 'bb$bb', 'cc()cc']

An even more concise variant of this is to pass the str.lower() method object right as a first argument of the map() function like so:

def all_lower(my_list):
    return list(map(str.lower,my_list))

If you struggle with the map() function, I’d recommend you check out the following video and blog tutorial:

Mastering the Python Map Function [+Video]

🌍 Recommended Tutorial: Python Map Function

Method 3: Simple For Loop and List Append 😖

The default but not so idiomatic way to convert a string list to a lowercase string list is to create an empty list and iterate over all elements in the original list, converting each element to a lowercase string using x.lower() and appending this element to the newly-created list.

Here’s a minimal example:

def all_lower(my_list):
    new_list = []
    for x in my_list:
        new_list.append(x.lower())
    return new_list


print(all_lower(['HELLO', 'Python', 'world']))
# ['hello', 'python', 'world']

print(all_lower(['A', 'B', 'C']))
# ['a', 'b', 'c']

print(all_lower(['AA AA', 'BB$BB', 'CC()CC']))
# ['aa aa', 'bb$bb', 'cc()cc']

It’s not so bad—mainly due to the efficiency of the list.append() method. Feel free to check out our full guide on list.append() with many interesting factoids such as which is faster: list.append() or list.extend()?

🌍 Recommended Tutorial: Python List Append Method

Summary

You’ve learned three main ways to convert a list of strings to a list of lowercase strings:

• [x.lower() for x in my_list]
• list(map(str.lower, my_list))
• Loop over all x and call list.append(x.lower())

In the process of solving this problem, we’ve learned many new Python features that every Python pro coder should know without using Google. 😉

If you want to keep improving your skills, I’ll invite you to join our free email academy and download our Python cheat sheets here:

---