if 语句

简单的示例：

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

for car in cars:
    if car == "bmw":
        print(car.upper())
    else:
        print(car.title())
1. 使用 使用and 检查多个条件 检查多个条件 
要检查是否两个条件都为True ，可使用关键字and 将两个条件测试合而为一；如果每个测试都通过了，整个表达式就为True ；如果至少有一个测试没有通过，整个表达式就 为Fals
2. 使用 使用or 检查多个条件 检查多个条件 
关键字or 也能够让你检查多个条件，但只要至少有一个条件满足，就能通过整个测试。仅当两个测试都没有通过时，使用or 的表达式才为False 
3.简单的if语句
if conditional_test:    
  do something
4.if-else 语句

ages = 17
if ages>=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!")
5.if-elif-else 结构

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.")



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

requested_toppings = ['mushrooms', 'extra cheese']
if 'mushrooms' in requested_toppings:
    print("Adding mushrooms.")
elif 'pepperoni' in requested_toppings:
    print("Adding pepperoni.")
elif 'extra cheese' in requested_toppings:
    print("Adding extra cheese.")
print("\nFinished making your pizza!")

6.使用多个elif代码块

age = 12
if age <4:
    price = 0
elif age <18:
    price =5
elif age <65:
    price = 10
else:
    price =5
print("Your admission cost is $" + str(price) + ".")

7.使用if语句处理列表

#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!")


#确定列表不为空
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?")



#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!")