05-Python-判断语句

1、条件测试

每条if语句的核心都是一个值为True或False的表达式，这种表达式被称为条件测试。Python根据条件测试的值为True还是False来决定是否执行if语句中的代码。条件测试为True，则执行；否则，不执行。

1.1、检查特定值是否包含在列表中

 

banned_users = ['andrew','carolina','david']
user = 'marie'

if user in banned_users:
    print(user.title() + ", you can post a response if you wish.")
if user not in banned_users:
    print(user.title() + " doesn't exist in list.")

1.2、布尔表达式

布尔表达式结果要么为True，要么为False。空字符串、列表、元组、字典等都为假。

bool([])  #值为False
bool({}) #值为False
bool([])   #值为False
bool("")  #值为False
bool(0)   #值为False

bool(1)   #值为True
bool(-1)  #值为True

1.3、if-elif-else

if <条件判断1>:
    <执行1>
elif <条件判断2>:
    <执行2>
elif <条件判断3>:
    <执行3>
else:
    <执行4>

#举例
age = int(input("please input your age: "))

if age < 4:  #4岁以下免费
    price = 0
elif age < 18:  #4~17岁5块
    price = 5
elif age < 65:  #18~64岁10块
    price = 10
else:  #大于等于65岁的半价
    price = 5

print("Your admission cost is $" + str(price) + ".")

1.4、测试多个条件

if-elif-else功能强大，但仅适合用于只有一个条件满足的情况：遇到通过了的测试后后，Python会跳过余下的测试。如果有时候需要关心所有的检测条件，则不能使用if-elif-else这种结构，而应该使用多个if语句。

#如果顾客点了两种配料，就需要确保在其披萨中包含'mushrooms'和'extra cheese'。
requested_toppings = ['mushrooms','extra cheese']

if 'mushrooms' in requested_toppings:
    print("Adding mushrooms.")
if 'pepperoni' in requested_toppings:
    print("Adding pepperoni.")
if 'extra cheese' in requested_toppings:
    print("Adding extra cheese:")

print("
Finished making your pizza!")

2、用if语句处理列表

2.1、处理特殊元素

requested_toppings = ['mushrooms','green peppers','extra cheese']

for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':  #如果点了青椒，则打印青椒没了。
        print("Sorry,we are out of green peppers right now.")
    else:
        print("Adding " + requested_topping + ".")

print("
Finished making your pizza!")

2.2、确定列表不是空的

requested_toppings = []

if requested_toppings:  #为真就执行循环体
　　for requested_topping in requeted_toppings: 
   　　print("Adding " + requested_topping + ".")
6　　 print("
Finished making your pizza!")
else:
　　print("You get a plain pizza!")

2.3、使用多个列表

available_toppings = ['mushrooms','olives','green peppers','pepperoni','pineapple','extra cheese']
requested_toppings = ['mushrooms','french fries','extra cheese']

for requested_topping in  requested_toppings:
    if requested_topping in available_toppings:
        print("Adding " + requested_topping + ".')
    else:
        print("Sorry, we don't have " + requested_topping + ".")

print("
Finished making your pizza!")