if语句可以根据用户的输入内容或获取的数据进行条件判断，以响应对应条件的代码块。

语法：if...else...

例子：

username = input("username:")
password = input("password:")
if username == "george" and password == "georgeNB":print("Welcome My Lord")
else:print('Wrong username or password！')

语法：if...elif...else 用于多条件判断。

例子一：

age = 56
while True:user_age = int(input('input your age:'))if user_age > age:print('try smaller')elif user_age < age:print('try big')else:user_age = "exit"print('your got it!')break

例子二：

'''
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'''
while True:score = int(input("input your score:"))if score == 100:print("your score 'A++'")elif 90 <= score <=95:print("your score 'A+' ")elif 80 <= score <= 89:print("your score 'A'")elif 70 <= score <=79:print("your score 'B+'")elif 60 <= score <=69:print("your score 'B'")elif score <=59:print("your score 'C'")else:score > 100print("Wrong data!")break

练习题

1、猜年龄，猜三次

age = 56
count = 0
while True:user_age = int(input('input your age:'))if user_age > age:count += 1print(count)print('try smaller')if count >2:breakelif user_age < age:count += 1print(count)print('try big')if count >2:breakelse:user_age = ageprint('your got it!')break

2、猜年龄，每隔三次问一次，是否继续(Y/N)

'''
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'''
age = 56
count = 0
while True:user_age = int(input('input your age:'))if user_age > age:count += 1print(count)print('try smaller')if count >2:print("Do you want to choose to try again(Y/N)")choose = input("input your choose:")if choose == "Y" or "y":count =0else:breakelif user_age < age:count += 1print(count)print('try big')if count >2:print("Do you want to choose to try again(Y/N)")choose = input("input your choose:")if choose == "Y" or "y":count = 0else:breakelse:user_age = ageprint('your got it!')break

3、三次登录(残缺版1.0beta)

username = "george"
password = "123"
count =0while True:input_username = input("input your username:")input_password = input("input your password:")if username == input_username and password == input_password:print("Welconme My Lord!")breakelse:count +=1print("Your username or password is Wrong!")if count>2:print("Your username or passwrod is Wrong three times, It is lock")break