Python基础知识：if语句

1、模拟网站确保用户名是否重复的方式，无视大小写，用到函数lower()

 #检查用户名是否重复
 current_users=['admin','alex','lebran','kaobi','James']
 lower_current_users=[current_user.lower() for current_user in current_users]#列表元素变为小写
 new_users=['Admin','alex','haden','Holliday','JAMES']
 for new_user in new_users:
     if new_user.lower() in lower_current_users:
         print('Input another username.')
     else:
         print('You can use it.')

2、if...and...条件测试

user="charlie"
passwd="xjl1991"
username=input("username:")
password=input("password:")
if user==username and passwd==password:
    print("欢迎登陆")
else:
    print("用户名或密码错误")

3、列表：先将列表清空，再检查列表是否为空

 list=['admin','alex','lebran','kaobi','james']
 list=[]  #将列表清空
 if list:
     print('h')
 else:
     print('We need to find some users!')

4、if-elif语句

# elif测试每个年龄段的收费情况
age=int(input('Please input your name:'))
if age < 4:
    price=0
elif age < 18:
    price=5
elif age < 65:
    price=10
else:
    price=5
print('Your admission cost is %s.'%price)#你的入场费是

5、三元运算，也叫三目运算，就是把简单的if--else--语句，合并成一句

#如果if后面的条件成立，就返回前面的值，不成立，返回后面的值
name = 'alex' if 1==1 else 'sb'

6、练习题：猜年龄游戏

age=26
counter=0
for i in range(10):
    print("--counter:",counter)
    if counter <3:
        guess_number=int(input("your guess number:"))
        if guess_number==age:
            print("Congretulations,you got it！")
            break
        elif guess_number>age:
            print("Think smaller.")
        else:
            print("Think bigger.")
    else:
        continue_confirm=input("do you want to try continue:")
        if continue_confirm=="y":
            counter=0
            continue
        else:
            break
    counter=counter+1