循环loop:
         1.有限循环
         2.无限循环(死循环)
         
         
for循环:
    #i是变量           #range(开始为x,结束为y,步长为z的循环)
        for i in range(x,y,z):  #把range循环赋给i      #range范围   
            print(i)  #输出i


例如:
       for i in range(2,6):  #步长Z为1可以默认不写;
           print(i)  
输出为:
       >>>[2,3,4,5]

例如:
       for i in range(1,10,2): #步长Z为2,即按2依次递进
           print(i)
输出为:
       >>>[1,3,5,7,9]


求1-100的奇数:
方法1:  
   for i in range(1,101):
        if i % 2 == 1:
            print("loop:",i)
方法2:
    for i in range(1,101,2)
        print(i)

求100的数,但除了50-70的数;
    for i in range(1,101):
        if i<50 or i>70:
            print(i,end=" ")
  

输入3次密码:

for i in range(3):

 _user   =  "leng"
 _passwd =  "wxc"


 username = input("username:")
 password = input("password:")


 if username == _user and password == _passwd:
    print("welcome %s login %s....."%(_user,_passwd))
    break;
 else:
    print("invalid username or password !")
    
进阶:
   
for i in range(3):
        _user   =  "leng"
        _passwd =  "wxc"

        panduan = False
        username = input("username:")
        password = input("password:")
    

        if username == _user and password == _passwd:
            print("welcome %s login %s....."%(_user,_passwd))
            panduan = True
            break
        else:
            print("invalid username or password !")
if not panduan:   #假的情况下,条件成立
    print("试太多了")
    
#if not:如果是假的,则输出X
    print(X)


一样进阶:

for i in range(3):
        _user   =  "leng"
        _passwd =  "wxc"

        username = input("username:")
        password = input("password:")
    
        if username == _user and password == _passwd:
            print("welcome %s login %s....."%(_user,_passwd))
            break   #break打断循环  for过后,就不会执行最后面的else语句
        else:
            print("invalid username or password !")
#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
else:
    print("试太多了")
#for后面只能加else while也一样