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