不要急于马上把三次登陆写出来，一定要将复杂的程序简单化，必须一步一步地去扩展，这样才保证不会出错。
# 事先定义
user = ‘dark_knight‘
pwd = ‘dk123‘
username = input(‘Please enter username:‘)
password = input(‘Please enter password:‘)
# 判断
if username == user and password == pwd:
print(‘Login successfully!‘)
else:
print(‘Login failed!‘)
# 事先定义
user = ‘dark_knight‘
pwd = ‘dk123‘
count = 1 # 定义次数
while True:
if count == 4: # 大于3次则退出循环
print(‘Too many times!‘)
break
username = input(‘Please enter username:‘)
password = input(‘Please enter password:‘)
# 判断
if username == user and password == pwd:
print(‘Login successfully!‘)
break
else:
print(‘Login failed!‘)
count += 1 # 每次循环都次数都进行加1操作
1、去除字符串两边的空格
2、当用户输入None时则提示用户反复输入。
3、当用户输入的信息不匹配时则告诉用户还有几次机会。
# 事先定义
user = ‘dark_knight‘
pwd = ‘dk123‘
count = 1 # 定义次数
while True:
if count == 4: # 大于3次则退出循环
print(‘Too many times!‘)
break
username = input(‘Please enter username:‘).strip() # 去除字符串两边的空格
password = input(‘Please enter password:‘).strip() # 去除字符串两边的空格
# 当用户输入有值时
if username and password:
# 判断
if username == user and password == pwd:
print(‘Login successfully!‘)
break
else:
print(‘Login failed!‘)
print(‘You have %s chance.‘%(3 - count))
# 当用户输入None时
else:
print(‘You enter blank, please re-enter！‘)
continue # 当用户输入None时，跳出本次循环。
count += 1 # 每次循环都次数都进行加1操作
原文：http://www.cnblogs.com/fyknight/p/7967436.html