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
来源：https://www.cnblogs.com/Black-sail/p/11544125.html