Properties are allowed to do whatever the heck they want. That doesn't mean that you have to execute code to determine whether they exist or not.
If you don't want to execute properties, do the lookup on the type, not the instance (obviously, you know the dance you need to do, since you've linked the code where you did it). Having apparently simple query operations like hasattr/getattr/len/etc execute arbitrary code is where much of the language's flexibility comes from, so I don't see how it can really be surprising when it happens. To me, Python's definition of an object having an attribute is "Object x has an attribute y if x.y does not raise AttributeError". That means it can't figure out whether or not the attribute exists without actually attempting to retrieve it. There are a few places where we instead use a heuristic that says an attribute *probably* exists if it appears in the instance dictionary, or in the dictionary of one of the types in the MRO, and magic methods have the rule that they must be defined on the type rather than the instance in order to count from the interpreter's point of view, but neither of those things change the basic definition. For the record, +1 on narrowing the scope of the exception suppression in hasattr() to only AttributeError, and adding new C API functions that expose the new behaviour. (I've actually long assumed that AttributeError *was* the only thing suppressed by hasattr()). Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia