UnboundLocalError Leetcode #5. Longest Palindromic Substring

Question

Leetocode keeps giving me this UnboundLocalError, and I don't know why...

This is my code

class Solution:
    def longestPalindrome(self, s: str) -> str:
        def isPalindrome(s):
            if len(s) == 1:
                return True
            if len(s) == 2 and s[0] == s[1]:
                return True
            else:
                if s[0] == s[-1]:
                    return isPalindrome(s[1:-1])
                else:
                    return False
        max_ = 0
        lenght = len(s)
        for i in range(lenght):
            for r in range(i + 1, lenght):
                if isPalindrome(s[i:r]):
                    len_ = r - i + 1
                    if len_ > max_:
                        max_ = len_
                        final = s[i:r]
        return final

and the error it gives me is

UnboundLocalError: local variable 'final' referenced before assignment
    return final

Can someone please help me understand why this may be occurring?

I think the problem may be given in the case where the final string is of len() = 1. In that case it might be a problem the s[i : r]

Answer 1

your final variable is defined inside the if block and is not visible outside. just declare it outside to get rid of this erro

class Solution:
    def longestPalindrome(self, s: str) -> str:
        def isPalindrome(s):
            if len(s) == 1:
                return True
            if len(s) == 2 and s[0] == s[1]:
                return True
            else:
                if s[0] == s[-1]:
                    return isPalindrome(s[1:-1])
                else:
                    return False
        max_ = 0
        lenght = len(s)

        final = 0 # declared final here

        for i in range(lenght):
            for r in range(i + 1, lenght):
                if isPalindrome(s[i:r]):
                    len_ = r - i + 1
                    if len_ > max_:
                        max_ = len_
                        final = s[i:r]
      

        return final

Answer 2

Testcase in which s consist of only one letter. like ("a","d") your inner for loop is not executing and hence you define final inside the if is not initialize and you are returning final hence unbound error occurs.

for ex. let's say s="s" Dry run-:

max_ :0

lenght :1 (i.e len(s))

for i in range(1):

for j in range(1,1) (i.e when first loop iterating i=0) no inner loop run

Exit()

return Final #Unbound error

To solve this error you can initialize final outside. or you can add a condition if len(s)==1: return 1

also your code solution has time complexity O(n^2) I think it will show Time limit exceeded.