Jun Wang added the comment:
I think this is a common problem while using both __getattr__ and
descriptor/property. A descriptor example:
class Descriptor():
def __get__(self, instance, owner=None):
raise AttributeError('Implicitly suppressed')
class A():
d = Descriptor()
def __getattr__(self, name):
return 'default'
print(A().d)
Without descriptor, unexpected AttributeError could only come from overriding
__getattribute__, which is a rare case, although still an imperfection. But in
descriptor/property, AttributeError which is too general just occurs frequently
like in normal method.
Surely any modification would break the backward compatibility, although I
wonder how often it is used of raising AttributeError purposely, maybe in
__getattribute__, to call __getattr__, instead of explicitly calling
__getattr__. In my understanding this is the only case that will be affected.
"An unexpected exception should not result in subtly altered behaviour, but
should cause a noisy and easily-debugged traceback. "—from PEP479
About the implementation, maybe something like "RuntimeError: descriptor raised
AttributeError" simulating PEP479. Or in my lay opinion, the best solution is:
add object.__getattr__, with the only behavior of raising AttributeError; when
normal attribute lookup fails, object.__getattribute__ calls __getattr__
explicitly; __getattr__ not triggered by AttributeError anymore.
I know little about the CPython implementation, so I might be completely wrong.
However this seems deserving more detailed discussion.
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<http://bugs.python.org/issue25634>
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