Python学习之路（5）if 语句

本章中，你将学习条件测试，学习简单的if语句，以及创建一系列复杂的if语句来确定当前到底处于什么情形。接下来将if应用于列表，以编写for循环。

if语句

一个简单实例：

cars = ['audi', 'bmw', 'subaru', 'toyota']
for car in cars:
    if car == 'bmw':
        print(car.upper())
    else:
        print(car.title())
#output
Audi
BMW
Subaru
Toyota

1.条件测试

• 等于（==），大于（>），小于（<）， 小于等于（<=），大于等于（>=）
• and、or
• 检查特定值是否包含在列表中（in）、是否不包含在列表中（not in）
• bool表达式（True、False）

2.使用if语句处理列表

以披萨店制作披萨为例，每添加一种配料都打印一条消息：

# usr if check list
requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
    print("Adding " + requested_topping + ".")
print("\nFinished making your pizza!")
#output
Adding mushrooms.
Adding green peppers.
Adding extra cheese.

Finished making your pizza!

青椒用光了：

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!")
#output
Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.

Finished making your pizza!

判断顾客点的配料列表是否为空：

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?")
#output:
Are you sure you want a plain pizza?

顾客的要求往往五花八门，下面定义两个列表。第一个包含披萨店供应的配料，第二个包含顾客点的配料。

requested_toppings = ['mushrooms', 'french fires', 'extra cheese']
available_toppings = ['mushrooms', 'olives', 'green peppers',
                      'pepperoni', 'pineapple', 'extra cheese']
for requested_topping in requested_toppings:
    if requested_topping in available_toppings:
        print("Adding " + requested_topping + ".")
    else:
        print("Sorry, we donot have " + requested_topping + ".")
print("\nFinished making your pizza!")
#output:
Adding mushrooms.
Sorry, we donot have french fires.
Adding extra cheese.

Finished making your pizza!