Python Sort List of Strings by Length

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💡 Problem Formulation: When working with lists of strings in Python, you may encounter a scenario where you need to sort the list not by alphabetical order, but by the length of the strings. For instance, given a list ["apple", "fig", "banana"], you’d want to reorganize the list to ["fig", "apple", "banana"], arranging the words by ascending length.

Method 1: Using sorted() with len()

The built-in sorted() function can be used to sort any iterable. When sorting a list of strings by length, you can pass len as the key argument, which instructs sorted() to sort by the length of the strings.

Here’s an example:

fruits = ["apple", "fig", "banana"]
sorted_fruits = sorted(fruits, key=len)
print(sorted_fruits)  
# Output: ["fig", "apple", "banana"]

In this snippet, sorted(fruits, key=len) tells Python to sort the fruits list, using the length of each string as the sorting key. The sorted list is ["fig", "apple", "banana"], ordered by ascending string length.

Method 2: Using the sort() Method with a Lambda Function

The sort() method sorts a list in place. If you want to sort a list by the lengths of its strings, you can use a lambda function as the key, which will be used to determine the strings’ lengths during the sort.

Here’s an example:

fruits = ["apple", "fig", "banana"]
fruits.sort(key=lambda fruit: len(fruit))
print(fruits)  
# Output: ["fig", "apple", "banana"]

By calling fruits.sort(key=lambda fruit: len(fruit)), we sort fruits in place, using a lambda function that returns the length of each string as the key. The list is modified to ["fig", "apple", "banana"], with shorter strings coming first.

Method 3: Using the sort() Method with len()

Similarly to the sorted function, the sort() method can also be used with len() to sort the list of strings by their lengths, manipulating the original list rather than creating a copy.

Here’s an example:

fruits = ["apple", "fig", "banana"]
fruits.sort(key=len)
print(fruits)  
# Output: ["fig", "apple", "banana"]

The expression fruits.sort(key=len) sorts the fruits list in place, using the length of the strings as the key for sorting. The manipulated list appears as ["fig", "apple", "banana"], sorted by ascending length.

Method 4: Using List Comprehension With Tuple Sorting

Python sorts tuples based on the first element by default. You can create a list of tuples, where the first element is the string length, and then sort it. Finally, use list comprehension to extract the sorted strings.

Here’s an example:

fruits = ["apple", "fig", "banana"]
fruits_with_length = [(len(fruit), fruit) for fruit in fruits]
fruits_with_length.sort()
sorted_fruits = [fruit for _, fruit in fruits_with_length]
print(sorted_fruits)  
# Output: ["fig", "apple", "banana"]

This code first pairs each string with its length in a tuple, sorts the list of tuples, and then strips away the lengths, leaving a list of strings sorted by length: ["fig", "apple", "banana"].

Bonus One-Liner Method 5: Using sorted() and a Generator Expression

For a concise one-liner, you can combine sorted() with a generator expression, which creates an iterable of tuples with string lengths and strings.

Here’s an example:

fruits = ["apple", "fig", "banana"]
sorted_fruits = [fruit for _, fruit in sorted((len(fruit), fruit) for fruit in fruits)]
print(sorted_fruits)  # Output: ["fig", "apple", "banana"]

This one-liner sorts a generator expression that produces tuples of string lengths and strings. The result is a new list of strings sorted by length: ["fig", "apple", "banana"].

Summary/Discussion

  • Method 1 (sorted() with len()): Returns a new sorted list; non-destructive.
  • Method 2 (sort() with Lambda): Sorts the list in place using a lambda function; efficient for long lists.
  • Method 3 (sort() with len()): Sorts the list in place; fewer keystrokes than a lambda function.
  • Method 4 (List Comprehension with Tuple Sorting): More complex; helpful when additional sorting conditions are needed.
  • Bonus Method 5 (One-Liner with sorted() and Generator): Extremely concise; best for those who prefer one-liners.

For immutable sorting, Method 1 is preferred, while Method 2 and 3 provide in-place sorting. Method 4 and the Bonus Method 5 offer a different syntax style but might be less readable for beginners.