5 Best Ways to Sort a List of Strings by Substring in Python

by

💡 Problem Formulation: You have a list of strings and you need to sort it based on a specific substring within each element. For example, if you have a list of files like ["foo_123.txt", "bar_456.txt", "baz_789.txt"] and you want to sort them by the numeric substring, the desired output should be ["foo_123.txt", "bar_456.txt", "baz_789.txt"] when sorted in ascending order.

Method 1: Using the sorted() Function with lambda

Python provides the sorted() function which, when combined with a lambda function, can sort a list of strings based on a substring. The lambda function extracts the target substring, which the sorted() function then uses as the key for sorting.

Here’s an example:

files = ["foo_123.txt", "bar_456.txt", "baz_789.txt"]
sorted_files = sorted(files, key=lambda x: x.split('_')[1])
print(sorted_files)

Output:

['foo_123.txt', 'bar_456.txt', 'baz_789.txt']

This code snippet sorts the list of file names by the numeric part which comes after the underscore. The lambda function splits each string at the underscore and takes the second element as the key for sorting.

Method 2: Using List Comprehensions and sorted()

List comprehensions in Python can also be used to sort strings by substring by constructing a list of tuples, where the first element is the substring and the second is the original string. The list of tuples can then be sorted with the sorted() function.

Here’s an example:

files = ["foo_123.txt", "bar_456.txt", "baz_789.txt"]
sorted_files = [file for _, file in sorted((x.split('_')[1], x) for x in files)]
print(sorted_files)

Output:

['foo_123.txt', 'bar_456.txt', 'baz_789.txt']

This code creates a sorted list of tuples, where each tuple contains the numeric substring and the original filename. The list comprehension then extracts just the filenames, sorted by the numeric substring.

Method 3: Custom Sort Function

A custom sort function can be defined using Python’s def keyword. This function can be intricately designed to control how the substrings are compared during the sorting process. The custom function is then passed to the sorted() function through the key argument.

Here’s an example:

def sort_key(string):
    return string.split('_')[1]

files = ["foo_123.txt", "bar_456.txt", "baz_789.txt"]
sorted_files = sorted(files, key=sort_key)
print(sorted_files)

Output:

['foo_123.txt', 'bar_456.txt', 'baz_789.txt']

The sort_key function takes a string and returns the substring used for sorting. The sorted() function uses this key function to order the list.

Method 4: Regular Expression Sorting

Python’s re module allows you to use regular expressions to define complex patterns for substrings. We can use this in conjunction with the sorted() function to sort strings by a matching pattern.

Here’s an example:

import re

files = ["foo_123.txt", "bar_456.txt", "baz_789.txt"]
sorted_files = sorted(files, key=lambda x: re.search(r'\d+', x).group())
print(sorted_files)

Output:

['foo_123.txt', 'bar_456.txt', 'baz_789.txt']

This snippet defines a lambda function that uses a regular expression to find all the digits in a string and uses that as the key for sorting.

Bonus One-Liner Method 5: Using operator.itemgetter()

The operator module provides a method itemgetter() which can be used to extract a sort key, similar to using a lambda. It is often faster and is a one-liner alternative to a lambda function.

Here’s an example:

from operator import itemgetter

files = ["foo_123.txt", "bar_456.txt", "baz_789.txt"]
sorted_files = sorted(files, key=itemgetter(slice(4, 7)))
print(sorted_files)

Output:

['foo_123.txt', 'bar_456.txt', 'baz_789.txt']

This line uses itemgetter to create a function that takes a slice of each string, starting from the 4th to the 7th character, to use as a sorting key. Suitable when the position of the substring within the string is well-known and consistent.

Summary/Discussion

  • Method 1: sorted() with lambda. Simple and inline. May lack performance with complex sorting logic.
  • Method 2: List Comprehensions and sorted(). Offers compact syntax. Less readable for beginners.
  • Method 3: Custom Sort Function. Highly customizable. Can become verbose for simple tasks.
  • Method 4: Regular Expression Sorting. Great for complex patterns. Overhead of regular expression processing.
  • Bonus Method 5: operator.itemgetter(). Efficient. Requires known substring indices; less flexible with variable patterns.