New submission from Martin Thurau: If you have a descriptor (in my case it was an SQLAlchemy column) on an instance and this descriptor returns None for a call to __get__ then getattr with a given default value, will not return the default, but None.
I have no knowledge on the implementation details of getattr but I guess the logic is something like this: - getattr looks at the given object and sees that the attribute in question is not None (since it is the descriptor object) - getattr returns the descriptor - the descriptors __get__ method is called - __get__ return None Maybe it should be more like this: - getattr looks at the given object and sees that the attribute in question is not None (since it is the descriptor object) - getattr sees that the attribute has __get__ - getattr calls __get__ method and looks if the return value is None I'm not sure if this is really a bug but it's highly confusing and somewhat un-pythonic. I really should not care of an attribute of an object is a value or a descriptor. This is especially true since this problem also applies to @property. Effectively this means that if you call getattr you have *know* if the name in question is a property or not and one can't simply swap out an objects value for a property without risking to break calling code. If this is actually *not* a bug, we should at least update the documentation to getattr, to mention this fact. Because currently it states that "getattr(x, 'foobar') is equivalent to x.foobar" which is obviously not true. ---------- components: Interpreter Core files: python_descriptor_bug.py messages: 212876 nosy: Martin.Thurau priority: normal severity: normal status: open title: getattr does not work well with descriptor type: behavior versions: Python 2.7 Added file: http://bugs.python.org/file34299/python_descriptor_bug.py _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue20864> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
