builtins.property not accepted as a valid type for subclassing

[beckjake]

mypy doesn't like it when I subclass the builtin class property:

class blah(property):
    pass

Running mypy 0.4.1 or 9e4cfc8 against that file, I get:
test.py:1: error: Invalid type "builtins.property"

But you totally can subclass property and build yourself a customized property decorator.

Looking at some debugger statements it seems like the type has been classified as not being an instance of TypeInfo. I'm afraid I can't offer a PR to fix this because I have no idea where that should be happening.

[gvanrossum]

Yeah, mypy probably only special-cases when the decorator is exactly 'builtins.property'. :-( Do you have a work-around?

[beckjake]

I thought that might be possible too, but changing to "builtins.property" also failed. The only place properties really seem to get handled is in SemanticAnalyzer.visit_decorator, but that never happens since this isn't a decorator.

I did come up with a work-around of just implementing my own descriptor, which works fine and probably is what I should have done in the first place, I just found it an odd curiosity.

[gvanrossum]

Ah, I'm wrong. The cause seems simpler: In builtins.pyi, both versions, the
definition of property is

property = object()

which makes it not a class.

I'd entertain a patch to typeshed if you're interested.

[ilevkivskyi]

The original example now passes mypy, not sure however how useful the subclass can be (property is heavily special-cased in mypy), but at least the original bug is fixed, so closing this.