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
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