Longest valid parentheses

class Solution:
    def longestValidParentheses(self, s: str) -> int:

        
        valid_chars = [False for _ in s]
        
        stack = []
        for i in range(0, len(s)):
            c = s[i]
            if c == '(':
                stack.append(i)
            elif c == ')' and len(stack) != 0:
                matching_i = stack.pop()
                valid_chars[i] = True
                valid_chars[matching_i] = True
            elif c == ')' and len(stack) == 0:
                pass
        
        longest_valid =0 
        current_valid = 0
        for c in valid_chars:
            if c:
                current_valid += 1
            else:
                longest_valid = max(current_valid, longest_valid)
                current_valid = 0
    
        longest_valid = max(current_valid, longest_valid)
        
        return longest_valid