TypeForm: Spelling for a type annotation object at runtime

[davidfstr (David Foster)]

@adriangb

Currently as per this proposal:

x = validate(Annotated[int, Gt(0)], 123)
reveal_type(x)  # int
assert x > 0  # type checkers do nothing

But if someone decided to support Gt(0) as recognized metadata in a type checker (which is allowed by PEP 593) then that type checker could do something like:

x = validate(Annotated[int, Gt(0), "foo"], 123)
reveal_type(x)  # Annotated[int, Gt(0)] ??
assert x > 0  # unnecessary assertion, I already know x > 0!

The current status quo is that static type checkers generally strip out the metadata part of an Annotated[T, metadata] type, such as in the code below:

x: Annotated[int, '> 0']
reveal_type(x)  # mypy: Revealed type is "builtins.int"
assert_type(x, int)

(Eric explains why in the comment starting with “No, metadata cannot be preserved.” in this thread.)

The current TypeForm proposal attempts to align with this status quo rather than change it, because there is limited benefit to changing the status quo for the applications that TypeForm is intended for in its Motivation.

Future proposals could very well alter the handling of Annotated[T, metadata] to preserve metadata. Or even introduce something entirely separate altogether, such as some kind of Refinement[...] syntax. But that’s out of scope for TypeForm.