5 Effective Methods to Sort a List of String Numbers Numerically in Python

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Problem Formulation

Sorting a list of string numbers numerically in Python can lead to unexpected issues.

For example, using the naive approach to sort the list lst = ["1", "10", "3", "22", "23", "4", "2", "200"] using lst.sort() will result in the incorrect order as it sorts the list of strings lexicographically, not numerically.

In this short article, my goal is to present the five best methods to correctly sort this list numerically. My recommended approach is the fifth one, see below. πŸ‘‡

Method 1: Convert Strings to Integers and Sort

This method involves converting each string in the list to an integer and then sorting them. It’s a direct and simple approach to ensure numerical ordering.

lst = [int(x) for x in lst]

Output: ['1', '2', '3', '4', '10', '22', '23', '200']

πŸ’‘ Recommended: Python List Comprehension

Method 2: Using the key Parameter with sort()

This method uses the key parameter with the int function to sort the strings as integers. It allows for numerical comparison without altering the original strings.


Output: ['1', '2', '3', '4', '10', '22', '23', '200']

πŸ’‘ Recommended: Python list.sort() with key parameter

Method 3: Using the natsort Module

The natsort module provides a natural sorting algorithm, useful for sorting strings that represent numbers. This method can handle more complex string sorting scenarios.

from natsort import natsorted
lst = natsorted(lst)

Output: ['1', '2', '3', '4', '10', '22', '23', '200']

Method 4: Using Regular Expressions

Using regular expressions, this method can sort strings containing both letters and numbers. It converts the numeric parts into floats for comparison, handling mixed content.

import re

def sort_human(l):
    convert = lambda text: float(text) if text.isdigit() else text
    alphanum = lambda key: [convert(c) for c in re.split('([-+]?[0-9]*\.?[0-9]*)', key)]
    return l

lst = sort_human(lst)

Output: ['1', '2', '3', '4', '10', '22', '23', '200']

πŸ’‘ Recommended: Python Regular Expression Superpower

Method 5: Using sorted() with key Parameter (Recommended)

This method combines the simplicity of using the key parameter with the benefit of creating a new sorted list, leaving the original untouched. It’s concise and effective.

lst = sorted(lst, key=int)

Output: ['1', '2', '3', '4', '10', '22', '23', '200']

πŸ’‘ Recommended: Python sorted() function

Summary – When to Use Which

  • Method 1: Converts strings to integers, then sorts. Simple but alters the original list.
  • Method 2: Uses the key parameter with int for sorting. Preserves the original strings.
  • Method 3: Utilizes the natsort module. Handles complex scenarios.
  • Method 4: Employs regular expressions for sorting alphanumeric strings.
  • Method 5 (Recommended): Combines the simplicity of using key with sorted(). Preserves the original list and offers concise code.

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