Simple palindrome function for single words

Question

I have created a simple palindrome function that works only on single words. It is working as intended, but I feel that it definitely needs improvement.

def is_palindrome(w):

    if not isinstance(w, str):
        return "ERROR: input is not a string."

    elif len(w) > 2:
        if w[0] == w[-1]:
            while len(w) > 2:
                w = w[1:-1]
                is_palindrome(w)

        if len(w) == 1:
            return True

        else:
            return False

    else:
        return "ERROR: input contains two or less characters."


print is_palindrome('aibohphobia')
print is_palindrome('definitely')

Can you please review my function and post your suggestions?

Also, is it better to do w[:] = w[1:-1] instead of w = w[1:-1]?

Answer 1

Firstly, your testing is insufficient. This code thinks that "defad" is a palindrome and that "deed" is not. You algorithm is completely broken.

def is_palindrome(w):

    if not isinstance(w, str):
        return "ERROR: input is not a string."

Don't check types. Just treat it like a string, and trust the caller to pass the right kind of object. Also don't return strings as errors, throw exceptions.

    elif len(w) > 2:
        if w[0] == w[-1]:
            while len(w) > 2:
                w = w[1:-1]
                is_palindrome(w)

You call the function but you don't do anything with the result.

        if len(w) == 1:
            return True

        else:
            return False

This is equivalent to: return len(w) == 1

    else:
        return "ERROR: input contains two or less characters."

Why can't palindromes be zero or one characters? Also, throw exceptions, don't return strings.

Answer 2

Why don't you check this way?

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

Answer 3

San4ez has the best implementation, and Winston Ewert raised valid points. I'd still like to add a few comments.

To answer your question, you should not use w[:] = w[-1] on strings, since this means trying to change your string, and strings are immutable in Python. w = w[-1] simply creates a new string, also named w.

Note than recursive functions are not Pythonic, since there is no tail call optimization, which means it would have been impossible for your recursive call to be optimized anyway. A simpler implementation:

def is_palindrome(w):
    while w and w[0] == w[-1]:
        w = w[1:-1]
    return w == ''

If you really wanted to go for the recursive way, you could have written:

def is_palindrome(w):
    return w == '' or w[0] == w[-1] and is_palindrome(w[1:-1])

Answer 4

You might try this function:

def is_palindrome(text):
    return text[:len(text)//2] == text[:(len(text)-1)//2:-1]

Here is example usage for reference:

>>> is_palindrome('')
True
>>> is_palindrome('a')
True
>>> is_palindrome('b')
True
>>> is_palindrome('aa')
True
>>> is_palindrome('ab')
False
>>> is_palindrome('ba')
False
>>> is_palindrome('bb')
True
>>> is_palindrome('aaa')
True
>>> is_palindrome('aab')
False
>>> is_palindrome('aba')
True