第05章 if 语句

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

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

输出结果：

You are old enough to vote!

2.2 if-else 语句

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!")

输出结果：

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

2.3 if-elif-else 结构

age = 12

if age < 4:
    price = 0
elif age < 18:
    price = 5
else:
    price = 15
    
print("Your admission cost is $" + str(price) + '.')

输出结果：

Your admission cost is $5.

还可以根据需要使用任意数量的elif代码块：

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) + '.')

输出结果：

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：

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) + '.')

输出结果：

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：

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('extra cheese.')
    
print('\nFinished 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

requested_toppings = ['mushrooms', 'extra cheese']

for requested_topping in requested_toppings:
	if requested_topping == 'green peppers':
    	print('Sorry, we are out of green peppers.')
	else:
    	print('Adding ' + requested_toppings + '.')
    
print('\nFinished making your pizza!')

输出结果：

Adding mushrooms.
Adding extra cheese.
Sorry, we are out of green peppers.

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：

requested_toppings = []

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

输出结果：

Are you sure you want a plain pizza?