循环

一：输出100以内的奇数：

for i in range(0,100):
    if i % 2 ==1:
        print(i)

for i in range(1,100,2):
    print(i)

二：输出100以内小于50大于70的数

for i in range(0,100):
    if i < 50 or i > 70:
        print('loop: ',i)

三：判断输入的用户名和密码，最多输入三次

_user = 'admin'
_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
    else:
        print('Invalid Username or Password')

_user = 'admin'
_passwd = '123'
passed_autheneication = True

for i in range(3):
    username = input('Username: ')
    password = input('Password: ')
    if username == _user and password == _passwd:
        print('Welcome %s login' % _user)
        passed_autheneication = False
        break
    else:
        print('Invalid Username or Password')

if passed_autheneication:
    print('操作过于频繁')

_user = 'admin'
_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
    else:
        print('Invalid Username or Password')

else:
    print('操作过于频繁')

_user = 'admin'
_passwd = '123'
i = 0

while i < 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')
    i += 1
    if i == 3:
        keep_going_chioce = input("还想继续吗？【Y/N】")
        if keep_going_chioce =='Y':
            i = 0