python3学习笔记6--条件判断if语句

• 一个简单示例

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

for car in cars:
    if car == 'bmw':
        print(car.upper())
    else:
        print(car.title())

• 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 。

1、检查是否相等

将变量当前值与特定值进行比较。

2、检查是否相等时不考虑大小写

当大小写不是很重要，可将变量的值转换为小写后比较。

3、检查是否不相等

判断两个值是否不相等，可结合使用感叹号和等号（!=）；!表示   不  。

4、比较数字

age = 18
if age == 18:
    print(age == 18)

5、检查多个条件

and or 

6、检查特定值是否包含在列表中

requested_toppings = ['mushrooms', 'onions', 'pineapple']
if 'mushrooms' in requested_toppings:
    print('mushrooms' in requested_toppings)

7、检查特定值是否不包含在列表中

not in

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

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

8、布尔表达式

条件测试的别名

• if 语句

age = 19
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")

age = 17
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")
else:
    print("Sorry, you are too young to vote.")
    print("Please register to vote as soon as you turn 18!")

age = 12 
if age < 4: 
    print("Your admission cost is $0.") 
    
elif age < 18: 
    print("Your admission cost is $5.") 
else:
    print("Your admission cost is $10.")

• 使用 if 语句处理列表

1、检查特殊元素

requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':
        print("Sorry, we are out of green peppers right now.")
    else:
        print("Adding " + requested_topping + ".")
print("\nFinished making your pizza!")

2、确定列表不是空

requested_toppings = [] 
if requested_toppings: 
    for requested_topping in requested_toppings: 
        print("Adding " + requested_topping + ".") 
    print("\nFinished making your pizza!") 
else:
    print("Are you sure you want a plain pizza?")

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("\nFinished making your pizza!")

• 设置 if 语句的格式

在条件测试的格式设置方面，PEP 8提供的唯一建议是，在诸如== 、>= 和<= 等比较运算符两边各添加一个空格，例如，if

age < 4: 要比if age<4: 好。 这样的空格不会影响Python对代码的解读，而只是让代码阅读起来更容易。