Feature request: support common metaclass usage

[Diggsey]

It's common to use meta classes as a way to define models, eg:

class Model(Base):
    my_column = Column(SomeType, ...)

This sort of pattern occurs in many places including ORMs and serialisation libraries, basically anything where a schema of some kind is needed.
What they all share is that the RHS of assignments in the class definition describe the type of those attributes on instances of the class, rather than the value of those attributes.

I think it would be worthwhile special-casing this usage: allow annotating a meta class with a type signature which tells mypy how to handle members. For the example above:

class BaseMeta(type):
    # field-generator: (Column[T]) -> T
    ...

This would give mypy the information it needs to determine the correct type of my_column.

[gvanrossum]

I've always assumed this would be a job for plugins.

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--Guido (mobile)

[Diggsey]

That seems reasonable, but there doesn't seem to be any documentation or examples about how to write plugins for mypy. I think I've read most of the documentation and until you mentioned it was unaware that mypy even supported plugins?

[JukkaL]

There's no way to write mypy plugins yet. We don't even have a concrete plan. See #1240 for some discussion.

[gvanrossum]

Let's close this as a dupe of the plugins issue (#1240).