def is_all_upper_or_lower(s): n = len(s) count_upper = 0 for i in range(n): if s[i].isupper(): count_upper += 1 if count_upper == 0 or count_upper == n: return True elif s[0].isupper() and count_upper == 1: return True else: return False while True: s = input("请输入用户名：") if True ==is_all_upper_or_lower(s): print("输入正确") else: print("输入错误")