PEP 695 TypeAliasType.__value__ not supporting recursion intentional?
#1816
Replies: 1 comment 4 replies
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I would recommend against using the I am not planning to change any of this behavior in the future (but that decision is not solely up to me). |
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@JelleZijlstra I found this issue based on the same assumption, that PEP 695 indicates a runtime-valid RHS of type parameters (even if not recursively expanded). A type GenericMetadata[T] = Annotated[T, list[T]]
type HardCodedMetadata = Annotated[int, list[int]]
print(HardCodedMetadata.__value__) # Annotated[int, list[int]]
print(GenericMetadata[int].__value__) # Annotated[T, list[T]] (not int??)It seems this behavior violates the |
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I don't know what to recommend to you because I don't know what you are trying to achieve. In general, I aim for the standard library to provide simple building blocks, and user code that knows what it wants can implement more complex operations (such as walking over a type and substituting type variables). |
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Thanks for the reply. My use case is to annotate IDs with information about what object they reference: type IdOf[T] = Annotated[UUID, T]This can be hooked by e.g. JSON schema generators in pydantic models to provide more schema information/typing to endpoints accepting UUID strings. As for how it relates to |
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Tools interpreting such metadata should recognize GenericAlias instances wrapping TypeAliasType instances and resolve the TypeVar accordingly. |
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I was moving some code to PEP695 and just realized an issue. Consider the following example:
and the supposed PEP 695 analogue fails:
The issue is that
Scalar.__value__is the Union of twoTypeAliasTypemembers. PEP 695 only states that "__value__is the evaluated value of the type alias", which under certain interpretations could be considered to mean that the result shouldn't contain anyTypeAliasType. The PEP also mentions__evaluate_value__in the context of PEP649, i.e. the resolution of Forward References.What's the recommended way of getting the value of the
TypeAliasTyperecursively? I guess for the moment, I could expand the unions manually. Will this be covered by__evaluate_value__in the future?Beta Was this translation helpful? Give feedback.
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