《Python编程：从入门到实践》笔记。
本章主要讲述条件语句if, if-else, if-elif, if-elif-else等结构。

1. 条件测试

包括了“相等”，“不等”，“大于”，“小于”，“大于等于”，“小于等于”，“存在于”，“与或非”等判断。值得注意的是，Python对大小写敏感：

>>> car = "Audi"
>>> car == "audi"
False

>>> car.lower() == "audi"
True

>>> car != "audi"
True

>>> age = 19
>>> age < 21
True
>>> age <= 21
True
>>> age >= 21
False

>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 and age_1 >= 21
False
>>> age_0 >= 21 or age_1 >= 21
True

>>> requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>> 'mushrooms' in requested_toppings
True
>>> 'mushrooms' not in requested_toppings
False

2. if 语句

2.1 简单的if语句

# 代码：
age = 19
if age >= 18:
    print("You are old enough to vote!")

# 结果：
You are old enough to vote!

2.2 if-else 语句

# 代码：
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!")

# 结果：
Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!

2.3 if-elif-else 结构

# 代码：
age = 12

if age < 4:
    price = 0
elif age < 18:
    price = 5
else:
    price = 10

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

# 结果：
Your admission cost is $5.

还可以根据需要使用任意数量的elif代码块：

# 代码：
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) + ".")

# 结果：
Your admission cost is $5.

其次，Python并不要求if-elif结构后面必须有else代码块。else是一条包罗万象的语句，只要不满足前面的条件，其中的代码就会执行，这可能会引入无效甚至恶意的数据。所以如果知道最终要测试的条件，应考虑使用一个elif代码块来代替else代码块，使代码更清晰，如下：

# 代码：
age = 12

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

# 结果：
Your admission cost is $5.

2.4 测试多个条件

if-elif-else结构功能强大，但仅适用于只有一个条件满足的情况，即只要其中一个条件满足，其余条件都会被跳过，这保证了程序的高效性。然而有时必须检查你关心的所有条件，这时则应该使用一系列不包含elif和else代码块的简单if语句：

# 代码：
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("\nFinished making your pizza!")

# 结果：
Adding mushrooms.
Adding extra cheese.

Finished making your pizza!

总之：如果你只想执行一个代码块，就用if-elif-else结构；如果要运行多个代码块，就使用一系列独立的if语句。

3. 使用if语句处理列表

if语句常和循环结构配合使用。

3.1 检查特殊元素

# 代码：
requested_toppings = ["mushrooms", "extra cheese", "green peppers"]

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!")

# 结果：
Adding mushrooms.
Adding extra cheese.
Sorry, we are out of green peppers right now.

Finished making your pizza!

3.2 确定列表不是空的

到目前为止，对于处理的每个列表都做了一个简单的假设，即它们非空，然而实际工程中，在遍历一个列表前需要先判断该列表是否为空：

# 代码：
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?")

# 结果：
Are you sure you want a plain pizza?

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