Python Palindromes One-Liner

5/5 - (1 vote)
"bob" == "bob"[::-1]

This one-liner introduces another basic computer science term: palindromes. Similar to anagrams, palindromes are a popular coding interview question.

First things first:

What is a Palindrome?

β€œA palindrome is a word, number, phrase, or other sequence of characters which reads the same backward as forward, such as madam or racecar or the number 10201.β€œ [source]

Here are a few fun examples:

  • β€œMr Owl ate my metal worm”
  • β€œWas it a car or a cat I saw?”
  • β€œGo hang a salami, I’m a lasagna hog”
  • β€œRats live on no evil star”
  • β€œHannah”
  • β€œAnna”
  • β€œBob”

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Problem Formulation

The problem is the following: Given a sequence of characters (string) – is the reverse sequence of characters the same as the original sequence (that is – is the string a palindrome)?

  • Whitespace matters, i.e., 'ann a' is not considered a palindrome, but 'anna' is.
  • Capitalization matters, i.e., 'Anna' is not considered a palindrome, but 'anna' is.

Python Palindrome Checker in One Line

## One-Line Palindrome Checker
is_palindrome = lambda phrase: phrase == phrase[::-1]

Listing: One-liner solution to check whether a phrase is a palindrome.

If you don’t like one-liners and lambda functions, you can also define an explicit function:

def is_palindrome(phrase):
    return phrase == phrase[::-1]

Let’s check how this works next.

How the Python Palindrome Checker Works

The one-liner solution does not need any external library, it’s simple and straightforward. We define a lambda function that takes a single argument phrase – the string to be tested – and returns a Boolean whether the sequence of characters remains unchanged when reversed. To reverse the string, we use slicing.

Let’s quickly recap slicing:

πŸ’‘ Slicing is a Python-specific concept for carving out a range of values from sequence types such as lists or strings. Slicing is based on the concise notation [start:stop:step] to carve out a sequence starting in index β€œstart” (inclusive) and ending in index β€œend” (exclusive). The third parameter β€œstep” allows you to define the step size, i.e., how many characters from the original sequence your slice will skip before taking the next character (e.g. step=2 means that your slice will consist of only every other character). When using a negative step size, the string is traversed in reverse order.

Thus, the result of the one-liner code snippet is the following:

## Result
# True

# False

print(is_palindrome("rats live on no evil star")) 
# True

Python Palindrome Checker Ignoring Whitespace and Capitalization

To check whether two strings are palindromes when ignoring capitalization and arbitrary whitespaces, first bring the string into a canonical form using ''.join(phrase.split()) and string.lower() to convert it to a lowercase string without whitespaces.

Then use the same expression string == string[::-1] to compare the string with its reverse representation using slicing with negative step size.

Here’s the code example:

def is_palindrome(phrase):

    # remove all whitespaces and lowercase string
    cleaned = ''.join(phrase.split()).lower()
    return cleaned == cleaned[::-1]

# True

print(is_palindrome("An\n\n     n \n\ta"))
# True

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2 thoughts on “Python Palindromes One-Liner”

    • Yeah, you’re right. It’s just an approximate fun palindrome… πŸ™‚
      Here’s how you could fix it:

      >>> s = “Mr Owl ate my metal worm”.lower().replace(” “,””)
      >>> print(s == s[::-1])

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