leetcode答案 有效的括号(python)

 def isValid(s):
     if s and len(s) % 2 is 0:
         a = {')': '(', ']': '[', '}': '{'}
         l = [None]
         for i in s:
             if i in a and a[i] == l[-1]:
                 l.pop()
             else:
                 l.append(i)
         return len(l) == 1
     elif len(s) % 2 is 1:
         return False
     else:
         return True