[New-bugs-announce] [issue20864] getattr does not work well with descriptor

[Martin Thurau]

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.

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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

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20864>
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