Python_Tips[4] -> and 和 or 的计算原则

and和or / and & or

---

对于and和or，可以连接多个值，其分别遵循原则:

1. 全是 And: 返回第一个遇到的无效值，若全有效，返回最后一个有效值
2. 全是 Or: 返回第一个遇到的有效值，若全无效，返回最后一个无效值
3. 混用 And & Or: 优先对所有 and 进行计算，随后计算所有 or，计算规则同上。

 print(1 and 0 and 2)    #
 print(1 and 2 and 3)    #
 print(0 or 1 or 0)      #
 print(0 or '' or None)  # None
 # Equal to (1 and 0) or (3 and 2 and 4) or (None and 5)
 print(1 and 0 or 3 and 2 and 4 or None and 5)   #

可以利用这些特性进行一些狡猾的处理，例如print出有效内容，内容无效则显示自定义的字符串。

可参考下面的代码

 """
 The return of 'and' and 'or'
 As for 'or', it will return the first valid one, if all invalid, return the last invalid one
 As for 'and', it will return the first invalid one, if all valid, return the last valid one
 """
 class Foo(): pass

 def foo(): pass

 valid = [7, True, (7,), [7], {'': 7}, foo, Foo]
 invalid = [0, None, False, '', (), [], {}]

 # 'or': Invalid or Valid --> return the first valid one
 print('----Below is Invalid or Valid----')
 for iv in invalid:
     print(iv or 'Valid' or 'Real')

 # 'or': Invalid or Invalid --> return the last invalid one
 print('----Below is Invalid or Invalid----')
 for iv in invalid:
     print(iv or None or False)

 # 'or': Valid or Valid --> return the first valid one
 print('----Below is Valid or Valid----')
 for vl in valid:
     print(vl or 'Valid')

 # 'and': Invalid and Valid --> return the first invalid one
 print('----Below is Invalid and Valid----')
 for iv in invalid:
     print(iv and 'Valid' and 'Real')

 # 'and': Valid and Valid --> return the last valid one
 print('----Below is Valid and Valid----')
 for vl in valid:
     print(vl and 7 and True)

 # 'and': Valid or Invalid --> return the first invalid one
 print('----Below is Valid and Invalid----')
 for vl in valid:
     print(vl and None and False)

 print('----Below is tricky print----')
 # We can use it to make a tricky print
 for iv in invalid:
     print('I have %s' % (iv or 'None'))
 for vl in valid:
     print('I have %s' % (vl or 'None'))

和输出结果

----Below is Invalid or Valid----
Valid
Valid
Valid
Valid
Valid
Valid
Valid
----Below is Invalid or Invalid----
False
False
False
False
False
False
False
----Below is Valid or Valid----
7
True
(7,)
[7]
{'': 7}
<function foo at 0x02F7F930>
<class '__main__.Foo'>
----Below is Invalid and Valid----
0
None
False

()
[]
{}
----Below is Valid and Valid----
True
True
True
True
True
True
True
----Below is Valid and Invalid----
None
None
None
None
None
None
None
----Below is tricky print----
I have None
I have None
I have None
I have None
I have None
I have None
I have None
I have 7
I have True
I have 7
I have [7]
I have {'': 7}
I have <function foo at 0x02F7F930>
I have <class '__main__.Foo'>