[Python-Dev] PEP 549: Instance Properties (aka: module properties) (original) (raw)

Larry Hastings larry at hastings.org
Sun Sep 10 23:52:12 EDT 2017

On 09/06/2017 02:13 PM, Ronald Oussoren wrote:

To be honest this sounds like a fairly crude hack. Updating the class of a module object feels dirty, but at least you get normal behavior w.r.t. properties.

Okay. Obviously I disagree, I think it's reasonable. But I'll assume you're -1.

Why is there no mechanism to add new descriptors that can work in this context?

I've updated the prototype to add one. I added it as "collections.abc.InstanceDescriptor"; that's a base class you can inherit from, and then your descriptor will work in a module. Bikeshedding the name is fine.

BTW. The interaction with import is interesting… Module properties only work as naive users expect when accessing them as attributes of the module object, in particular importing the name using “from module import prop” would only call the property getter once and that may not be the intended behavior.

I spent a fair amount of time thinking about this. The short answer is: we /could/ fix it. We /could/ make it so that "from x import y", when x.y is an instance descriptor, ensures that y honors the descriptor protocol when referenced. We'd have to do it for three contexts:

The first two are pretty similar; create a proxy object that retains the module instance so it remains "bound" to that. The third one is trickier; it'd mean a new bytecode (LOAD_FAST_DESCRIPTOR), but it wouldn't slow down people who didn't use it.

Anyway, long story short, I think this would be worse than simply having "from x import y" only calling the getter once. As the Zen says: special cases aren't special enough to break the rules.

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