range的使用：注意，在python3中，交互模式下已经不显示了

for循环的使用

打印50-70

# 第一种方案

for i in range(100):

    if i <= 70 and i >= 45:   #注意这里使用的是 and/or/not  非java中的&&，||,!

        print (i)

# 第二种方案

for i in range(50,71):

    print (i)

break/continue的使用

exit_Flag = False

for i in range(100):

    if i < 5:

        continue

    print("L_1:",i)

    for j in range(10):

        print("layer2:",j)

        if j == 7:

            exit_Flag=True

            break

    if exit_Flag:

        break

用户登录小程序

_user="hhh"

_pass="hello"

count = 1

while count <= 3:

    username = input("Please input the username:")

    password = input("Please input the password:")

    if _pass == password and _user == username:

        print("Welcome to BBT,", username)

        break

    else:

        print()

        print("Invalid username or password,you still have " + str(3 - count) + " times")

    count += 1

else:

    print("Time out,~-~")