[Python-ideas] Re: Introduce a boundary parameter for typing.get_type_hints when used with a class object

On 04.08.20 22:05, Guido van Rossum wrote:

Maybe get-type-hints can be refactored to make writing such a function simpler. IIRC the part that takes a single annotation and evaluates it is a private function.

That's what I considered and indeed it relies on `typing._eval_type`. If this was public then all building blocks would be in place.

On Tue, Aug 4, 2020 at 12:57 David Mertz mailto:mertz@gnosis.cx> wrote:

This definitely feels to me like though if an oddball case that "write your own function" seems like the best solution. I accept the OP needs it, but I have trouble imagining that many others would.

On Tue, Aug 4, 2020, 7:42 AM Dominik Vilsmeier mailto:dominik.vilsmeier@gmx.de> wrote:

In one of my projects I'm reusing class-level type annotations to identify relevant attributes for serialization, e.g. similar to the following:

    attrs = {name: getattr(obj, name) for name in get_type_hints(type(obj))}

This is convenient because it merges the type annotations from the various stages in the class hierarchy, e.g.

    class Base:         a: int     class Derived(Base):         b: str

results in `attrs == dict(a=..., b=...)`.

However it becomes inconvenient if external base classes are involved that define their own, unrelated type annotations, e.g.

    class External:  # from some other distribution         unrelated: float     class Base(External):         a: int

It would be helpful if `get_type_hints` had a `boundary` parameter that, when used with a class object, determines the upper boundary for the MRO. So it could be used in the following way:

    get_type_hints(type(derived_obj), boundary=Base)

to exclude any type annotations further up the class hierarchy (including the ones from `Base`).

Regarding the implementation this would effectively skip over base classes in the reverse MRO until it reaches the `boundary`.

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-- --Guido (mobile)

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