Ammar Askar <am...@ammaraskar.com> added the comment:
As unfortunate as this is, I don't think there's an easy way to solve this while adhering to descriptors and the attribute lookup model. This is a consequence of the following rule: > object.__getattr__(self, name): > Called when the default attribute access fails with an > AttributeError (either __getattribute__() raises an AttributeError > because name is not an instance attribute or an attribute in the > class tree for self; or __get__() of a name property raises > AttributeError) As it notes, if the __get__() raises an AttributeError then a fallback to __getattr__ is initiated. One may think that maybe we can just catch AttributeError in @property to try to fix this problem but a quick search shows that people do intentionally raise AttributeError in @property methods: * https://github.com/kdschlosser/EventGhost-TPLink/blob/a4a642fde8dd4deba66262a36d673cbbf71b8ceb/TPLink/tp_link/rule.py#L148-L152 * https://github.com/ajayau404/sniffer/blob/cd0c813b8b526a3c791735a41b13c7677eb4aa0e/lib/python3.5/site-packages/vpython/vpython.py#L1942-L1944 While this in combination with a __getattr__ is rare, I was able to find one example: * https://github.com/xrg/behave_manners/blob/19a5feb0b67fe73cd902a959f0d038b905a69b38/behave_manners/context.py#L37 I don't think that changing this behavior is acceptable as people might be relying on it and it's well documented. In your case, I think it's really the "catch-all" __getattr__ that's at fault here which really shouldn't be returning None for all attributes blindly. This does bring up a good point though that in the process of this fall-back behavior, the original AttributeError from A's property does get masked. What can be done and might be a good idea is to show the original AttributeError failure as the cause for the second. Something like this: Traceback (most recent call last): File "<stdin>", line 2, in <module> File "<stdin>", line 5, in myprop AttributeError: 'int' object has no attribute 'foo' The above exception was the direct cause of the following exception: Traceback (most recent call last): File "<stdin>", line 7, in <module> File "<stdin>", line 5, in <module> File "<stdin>", line 7, in __getattr__ AttributeError: not found in __getattr__ This at least somewhat indicates that the original descriptor __get__ failed and then __getattr__ also raised. As opposed to now where the original exception gets masked: >>> class A: ... @property ... def myprop(self): ... a = 1 ... a.foo ... def __getattr__(self, attr_name): ... raise AttributeError("not found in __getattr__") ... >>> a = A() >>> a.myprop Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 7, in __getattr__ AttributeError: not found in __getattr__ I'm gonna take a stab at implementing this real quick to see if it's actually useful and viable. ---------- nosy: +ammar2 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue39865> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
