[Tutor] What protocol to follow when need to pick either one from __getattr__ and __getattribute__ ?

[Arup Rakshit]

I read today 2 methods regarding the customizing the attribute 
access:__getattr__ and __getattribute__ from 
https://docs.python.org/3/reference/datamodel.html#special-method-names. 
What I understood about them is that __getattr__ is called when the 
requested attribute is not found, and an AttributeError is raised. But 
later is called everytime unconditionally. I wrote a simple 2 input 
calculator program, where only 2 operations are permitted Addition and 
Subtraction. Anything else will cause an not permitted error.

class OperationNotPermitted(AttributeError):
     pass

class Calc:
     def __init__(self, x, y):
         self.x = x
         self.y = y

     def __getattr__(self, name):
         if name == "sum":
             return self.x + self.y
         elif name == 'minus':
             return self.x - self.y
         else:
             raise OperationNotPermitted("operation {} is not 
permitted".format(name))

And here is a run down:

from customize_attr_access import *
cal = Calc(12, 10)
cal.sum
22
cal.minus
2
cal.mul
Traceback (most recent call last):
   Python Shell, prompt 5, line 1
     # Used internally for debug sandbox under external interpreter
   File "/Users/aruprakshit/python_playground/customize_attr_access.py", 
line 15, in __getattr__
     raise OperationNotPermitted("operation {} is not 
permitted".format(name))
customize_attr_access.OperationNotPermitted: operation mul is not permitted

If I replace __getattr__ with __getattribute__ I found the program works 
exactly same. Now my questions is in real world when  you have to pick 
between these 2 pair of special method which protocols a Python dev 
checks to pick either of the one? Is there any such thing, or either one 
is fine. Can anyone elaborate this to educate me please?

doc said:

 > This method should either return the (computed) attribute value or 
raise an AttributeError exception.

Another question:

My question is that: Can I raise a domain error like 
OperationNotPermitted when raising instead of AttributeError ?

-- 
Thanks,

Arup Rakshit