Python学习（七）if语句

Python if语句

Python中，if语句可以检查程序的状态，并依据状态采取相应的措施

cars = ['audi','bmw','subru','toyota']

      for car in cars:

          if car == 'bmw':

              #car等于“bmw"全部大写

              print(car.upper())

          else:

              #首字母大写

              print(car.title())

Audi

BMW

Subru

Toyota

条件测试

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，这种表达式称为条件测试。

检查是否相等

>>> car = 'bmw'
>>> car == 'bmw'
True
>>> car == 'Bmw'
False

检查是否相等时不考虑大小写

不考虑大小写时可将单词转换为全部小写

>>> car = 'Bmw'
>>> car == 'bmw'
False
>>> car.lower() == 'bmw'
True

name = "Hello world!"

单词首字母大写

print(name.title())

Hello World!

单词全部字母大写

print(name.upper())

HELLO WORLD!

单词全部字母小写

print(name.lower())

hello world!

检查是否不相等

判断两个值是否不等，可以结合使用感叹号和等号（!=）,很多程序语言都是这样表示

requested_topping = 'mushrooms'

  if requested_topping != 'anchovies':

      print("Hold the anchovies")

  else:

      print("Hold not the anchovies")

Hold the anchovies

比较数字

数字对比可以使用==（=，>,>=,<,<=,!=）==

>>> age = 18
>>> age == 18
True
>>> age = 19
>>> age < 20
True
>>> age <= 20
False
>>> age >= 20
False

检查多个条件

1、使用and检查多个条件

检查多个条件都符合，使用and

age_0 = 22
age_1 = 18
print(age_0 >= 21 and age_1 >=21)

False

age_1 = 21
print(age_0 >= 21 and age_1 >=21)

True

2、使用or检查多个条件

检查多个条件只要一条符合，使用or

age_0 = 22
age_1 = 18
print(age_0 >= 21 or age_1 >=21)
True
age_0 = 18
print(age_0 >= 21 or age_1 >=21)
False

检查特定值是否包含在列表中

检查特定值是否包含在列表中，实例如某用户在BBS中已经被禁言，不允许在发表评论。这是就可以使用“not in”筛选这个用户

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

user = 'marie'

if user not in banned_users:

print("%s, you can post a response if you wish."%(user.title()))
Marie, you can post a response if you wish.

布尔值表达

布尔值表达：相同“True”；不同“False”

确定列表不是空的

requested_toppings = []
if requested_toppings:
    for i in requested_toppings:
        print('adding %s .'%(i))
    print("==>>: Finished making your pizza!")
else:
    print("==>>: Are you sure you want a plain pizza?")
    
==>>: Are you sure you want a plain pizza?
列表为空不执行for循环直接执行else

使用多个列表对比

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!")
    
    Adding mushrooms.
    Sorry, we don't have french fries.
    Adding extra cheese.
    
    Finished making your pizza!