On 24/05/2023 15:37, A KR wrote:
It is perfectly explained in the standards here [1] saying that:
<quote>
In order to avoid infinite recursion in this method, its implementation should
always call the base class method with the same name to access any attributes
it needs, for example, object.__getattribute__(self, name).
</quote>
Therefore, I wrote a code following what the standard says:
<code>
class Sample():
def __init__(self):
self.a = -10
def __getattribute__(self, name):
if name == 'a':
return object.__getattribute__(self, name)
raise AttributeError()
s = Sample()
result = s.a
print(result)
</code>
I did not fall into recursion, and the output was
-10
While this works it's not how I understand the recommended pattern. I'd
rather treat "special" attributes first and then use the
__getattribute__ method of the base class as a fallback:
>> class Demo:
def __getattribute__(self, name):
if name == "answer":
return 42
return super().__getattribute__(name)
That way your special arguments,
>>> d = Demo()
>>> d.answer
42
missing arguments
>>> d.whatever
Traceback (most recent call last):
File "<pyshell#13>", line 1, in <module>
d.whatever
File "<pyshell#10>", line 5, in __getattribute__
return super().__getattribute__(name)
AttributeError: 'Demo' object has no attribute 'whatever'
and "normal" arguments are treated as expected
>>> d.question = "What's up?"
>>> d.question
"What's up?"
Eventual "special" arguments in the superclass would also remain accessible.
However, when I try the code without deriving from a class:
class AnyClassNoRelation:
pass
class Sample():
def __init__(self):
self.a = -10
def __getattribute__(self, name):
if name == 'a':
return AnyClassNoRelation.__getattribute__(self, name)
raise AttributeError()
s = Sample()
result = s.a
print(result)
and calling __getattribute__ via any class (in this example class
AnyClassNoRelation) instead of object.__getattribute__(self, name) as the
standard says call using the base class, I get the same output: no recursion
and -10.
So my question:
How come this is possible (having the same output without using the base
class's __getattribute__? Although the standards clearly states that
__getattribute__ should be called from the base class.
AnyClassNoRelation does not override __getattribute__, so
>>> AnyClassNoRelation.__getattribute__ is object.__getattribute__
True
There is no sanity check whether a method that you call explicitly is
actually in an object's inheritance tree,
>>> class NoRelation:
def __getattribute__(self, name):
return name.upper()
>>> class Demo:
def __getattribute__(self, name):
return "<{}>".format(NoRelation.__getattribute__(self, name))
>>> Demo().some_arg
'<SOME_ARG>'
but the only purpose I can imagine of actually calling "someone else's"
method is to confuse the reader...
<quote>
In order to avoid infinite recursion in this method, its implementation should
always call the base class method with the same name to access any attributes
it needs, for example, object.__getattribute__(self, name).
</quote>
Literally, I can call __getattribute__ with anyclass (except Sample cause it
will be infinite recursion) I define and it works just fine. Could you explain
me why that happens?
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