Python：简单的 If-Else 语句不起作用

Question

I'm coding a dynamic programming function that finds the longest substring that's a palindrome in Python:

def longestPalindrome(s: str) -> str:
        dict = {}
        start = 0
        end = 0

        # initialize base cases and left of base cases
        for i in range(len(s)):
            dict[(i,i)] = 1

        for y in range(len(s), -1, -1):
            for x in range(y, len(s)):
                print((x,y))
                # memo check
                if (x, y) not in dict.keys():
                    if s[x] == s[y]:
                        if x - y == 1:
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x

                        elif dict[(x-1,y+1)] == 1: # if substr betweet x & y is palindrome
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x                        
                    else:      # ISSUE HERE
                        dict[(x,y)] = 0

        return s[start:end+1]

I get the following error when inputting "aaabaa" into the function: KeyError: (4, 1)

When I make the following changes, it outputs correctly

def longestPalindrome(s: str) -> str:
        dict = {}
        start = 0
        end = 0

        # initialize base cases and left of base cases
        for i in range(len(s)):
            dict[(i,i)] = 1

        for y in range(len(s), -1, -1):
            for x in range(y, len(s)):
                print((x,y))
                # memo check
                if (x, y) not in dict.keys():
                    dict[(x,y)] = 0  # **CHANGE**
                    if s[x] == s[y]:
                        if x - y == 1:
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x

                        elif dict[(x-1,y+1)] == 1: # if substr betweet x & y is palindrome
                            dict[(x,y)] = 1
                            if (end - start) <= (x-y):
                                start = y
                                end = x                        
#                     else:         # **REMOVE**
#                         dict[(x,y)] = 0

        return s[start:end+1]

I'm pretty sure the two versions of the functions are logically the same, but I cannot figure out why the first doesn't work.

Answer 1

Did you intend to place the else one tab width back? Your else ought to be matching with the memoization step. It is matching with if s[x] == s[y]: