largest palindrome

def largest_palindrone(string):
    if not string:
        return ''
    else:
        longest = ['']
    lp_helper(string, longest)
    return longest

def memo(func):
    cache = {}
    cache2 = {}
    def memoize(string, longest):
        if len(longest) == 0:
            key = string
        else:
            key = string + longest[0]
        if not key in cache:
            cache[key] = func(string, longest)
            cache2[key] = longest[0]
        longest[0] = cache2[key]
        return cache[key]
    return memoize

@memo
def lp_helper(string, longest):
    if not string:
        return True
    if len(string) == 1:
        if not len(longest[0]):
            longest[0] = string
        return True
    if string[0] == string[-1]:
        if lp_helper(string[1:len(string)-1], longest):
                if len(longest[0]) < len(string):
                    longest[0] = string
                return True
        else:
            return False
    else:
        substr_has_pal = lp_helper(string[1:], longest) or lp_helper(string[0:len(string)-1], longest)
        return False