for 循环实例

print 1 至 100的奇数：

#print 奇数
for i in range(1,101,2) :　　# 2 为步长
    print("loop:",i)

打印小于50和大于70的数字：

for i in range(100):
    if i < 50 or i > 70:
        print(i)

三次输入用户信息：

_user = "eva"
_passwd = "123"

for i in range(3):
    username = input("Username:")
    password = input("Password:")

    if username == _user and password == _password:
        print("Welcome %s login..." %_user)
        break
    else:
        print("Invalid username or password!")

 三次输入不成功后打印 You can't try any more!：

_user = "eva"
_passwd = "123"

passed_authentication = False   #默认假，不成立（flag, 标志位）

for i in range(3):
    username = input("Username:")
    password = input("Password:")

    if username == _user and password == _passwd:
        print("Welcome %s login..." %_user)
        passed_authentication = True   #真，成立
        break
    else:
        print("Invalid username or password!")

if not passed_authentication:
    print("You can't try any more！")

或者执行以下代码：

_user = "eva"
_passwd = "123"



for i in range(3):
    username = input("Username:")
    password = input("Password:")

    if username == _user and password == _passwd:
        print("Welcome %s login..." %_user)
        break   #break for后就不会执行最后那个else后的语句
    else:
        print("Invalid username or password!")
else:     #只要上面的for循环正常执行完毕，中间没被打断，就会执行else语句
    print("You can't try any more！")

或者执行以下代码：

_user = "eva"
_passwd = "123"

counter = 0

while counter < 3:

    username = input("Username:")
    password = input("Password:")
    if username == _user and password == _passwd:
        print("Welcome %s login..." %_user)
        break   
    else:
        print("Invalid username or password!")

    counter += 1
else:
    print("You can't try any more!")