5 Best Ways to Find Out if Strings Differ by a Single Character in Python

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πŸ’‘ Problem Formulation: The task is to compare two strings and determine if they differ by exactly one character at the same position. For instance, comparing ‘spot’ with ‘spit’ shows that these strings differ by the third letter, ‘o’ in the former and ‘i’ in the latter. The goal is to identify such differences efficiently in Python.

Method 1: Iterative Comparison

This method involves iterating through both strings simultaneously and comparing characters at the corresponding positions. The function specification may return the index where the difference occurs or a boolean indicating whether a single difference was found.

Here’s an example:

def find_single_difference(str1, str2):
    if len(str1) != len(str2):
        return False
    diff_count = 0
    for i in range(len(str1)):
        if str1[i] != str2[i]:
            diff_count += 1
            if diff_count > 1:
                return False
    return diff_count == 1

print(find_single_difference('python', 'pythxn'))

Output: True

This snippet defines a function that compares two strings of equal length character by character. If a character difference is found, a count is incremented. If more than one difference is found, the function immediately returns False. Finally, it checks if there has been exactly one difference to return True.

Method 2: Zipping and Counting

This method uses the built-in zip() function in Python to pair up characters of both strings and then counts the differing pairs. It’s efficient and Pythonic in utilizing built-in functions.

Here’s an example:

def find_single_difference_zip(str1, str2):
    return sum(1 for a, b in zip(str1, str2) if a != b) == 1

print(find_single_difference_zip('hello', 'hallo'))

Output: True

The function find_single_difference_zip() zips the two strings and uses a generator expression with a conditional if statement to count the non-matching character pairs, which must equal one for the function to return True.

Method 3: List Comprehension and Sum

Similar to the zipping method, but instead of using a generator expression, this method leverages list comprehension alongside sum() to find the number of differing characters. This method is concise and readable.

Here’s an example:

def find_single_difference_listcomp(str1, str2):
    return sum([a != b for a, b in zip(str1, str2)]) == 1

print(find_single_difference_listcomp('book', 'boon'))

Output: True

The function find_single_difference_listcomp() creates a list of boolean values indicating where the characters differ and sums them up, hoping for a sum of one to confirm a single character difference.

Method 4: Using a Counter

This approach involves using the collections.Counter from Python’s standard library to count differing characters. While it is a more general approach, it can be overkill for this specific problem but demonstrates the flexibility of Python’s libraries.

Here’s an example:

from collections import Counter

def find_single_difference_counter(str1, str2):
    cnt = Counter(str1[i] != str2[i] for i in range(len(str1)))
    return cnt[True] == 1

print(find_single_difference_counter('shot', 'slot'))

Output: True

By using a Counter, this function tallies the number of True values indicating differences between the strings. If there is exactly one True, it signifies a single character difference.

Bonus One-Liner Method 5: Using Enumerate and a Generator Expression

A one-liner approach can highly compact the logic using enumerate and a generator expression. This method provides a Pythonic and efficient way to solve the problem with a single line of code.

Here’s an example:

find_single_difference_oneliner = lambda str1, str2: sum(1 for i, (a, b) in enumerate(zip(str1, str2)) if a != b) == 1

print(find_single_difference_oneliner('mate', 'male'))

Output: True

This lambda function utilizes enumerate() and zip() in a compact expression to calculate the sum of character differences. It checks to ensure this sum equals one for a single difference.


  • Method 1: Iterative Comparison. Simple, doesn’t require imports, but is a bit verbose. Good for teaching basic concepts.
  • Method 2: Zipping and Counting. Elegant and utilizes Python’s iterator concepts effectively. Perhaps too concise for new programmers to understand at first glance.
  • Method 3: List Comprehension and Sum. Offers clear intent and is easy to read. However, it generates an intermediate list, which can be a concern for very long strings.
  • Method 4: Using a Counter. Demonstrates the power of Python’s library but is less straightforward than other methods. Overhead of importing a library and creating a Counter object not needed for the problem.
  • Method 5: Bonus One-Liner. Quite Pythonic and concise, making it elegant for those comfortable with Python’s functional aspects. May sacrifice some readability for brevity.