python中的if语句

条件测试

条件测试别名布尔表达式
1. 条件测试
1）python检查是否相等时，区分大小写

2）使用and/or检查多个条件

3）检查特定值是否包含在列表中

2. if语句

if conditional_test:
	do something 

3. if-else语句

if conditional_test:
	do something
else:
	do something 

注：条件通过执行第一个缩进语句，条件不通过执行下一缩进语句

4. if-elif-else语句

if conditional_test:
	do something
elif conditional_test2:
	do something
elif conditional_test3:
	do something
else: 
	do something

注：1. 第一个elif下语句的执行条件，不符合conditional_test ，但符合conditional_test2，elif的执行仅当满足测试条件时才会执行。
2. else是一条保罗万象的语句，只要不满足任何if，elif中的条件测试，其中代码就会执行，这可能会引入无效或恶意数据，如果明确最终测试条件，可以考虑用elif代替else。
3. if-elif-else语句仅适用于只有一个条件满足的情况：即当有一条测试通过后，python会跳过其他测试执行语句
4. 测试多个条件
eg：餐厅点单时检查顾客菜单，多个判断条件叠加

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.')

Adding mushrooms.
Adding extra cheese.

Finished making your pizza.

如果使用if-elif-else语句，代码将不能正常运行

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.')

Adding mushrooms.

Finished making your pizza.

5. 使用if语句处理列表
假设菜单是固定我们可以用元组处理，菜单不固定我们可以使用列表

requested_toppings = ['mushrooms', 'extra cheese']
menu = ('mushrooms', 'pepperoni', 'extra cheese')
print(menu)
print(requested_toppings)
for requested_topping in requested_toppings:
    if requested_topping not in menu:
        print('Sorry, we are out of ' + requested_topping + ' now.')
    else:
        print('Adding '+ requested_topping + '.')
print('Finished making your pizza.')
print('\nend')

('mushrooms', 'pepperoni', 'extra cheese')
['mushrooms', 'extra cheese']
Adding mushrooms.
Adding extra cheese.
Finished making your pizza.

end

6. 如何确定列表不为空

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?')
print(bool(requested_toppings))

Are you sure you want a plain pizza?
False