I propose that we just clarify this in the docs we'll write for TypeGuard.
On Sat, Apr 10, 2021 at 8:33 AM Eric Traut <eri...@microsoft.com> wrote:
> Hi Barry,
>
>
>
> Thanks for the note. Apologies for the slow reply — your email got trapped
> in Microsoft’s spam filters, and I just noticed it.
>
>
>
> The idea of using a wrapper type was my first thought too. In fact, I
> implemented that solution in prototype form. It was disliked by almost
> everyone who tried to use the feature. The wrapper approach also got a
> negative reaction on the typing-sig when I posted the initial proto-spec. A
> wrapper prevents some common use cases (e.g. filter functions) and was
> found to be cumbersome and confusing.
>
>
>
> I understand your concern about the fact that type guards return bools but
> this is not reflected in the return type. This was debated at length in the
> typing-sig, and we considered various alternatives. In the end, we weren’t
> able to come up with anything better. I’m somewhat comfited by the fact
> that TypeScript’s formulation of this feature (which was the inspiration
> for the idea and is generally a well-liked feature in that language) also
> does not directly mention “boolean” in its return type annotation. Here’s
> an example of the syntax in TypeScript:
>
>
>
> ```
>
> function isNone(type: Type): type is NoneType {
>
> return type.category === TypeCategory.None;
>
> }
>
> ```
>
>
>
> -Eric
>
>
>
>
>
> On 4/6/21, 1:31 PM, "Barry Warsaw" <ba...@python.org> wrote:
>
> The Python Steering Council reviewed PEP 647 -- User-Defined Type Guards,
> and is happy to accept the PEP for Python 3.10. Congratulations Eric!
>
>
>
> We have one concern about the semantics of the PEP however. In a sense,
> the PEP subverts the meaning of the return type defined in the signature of
> the type guard, to express an attribute of the type guard
> function. Meaning, type guard functions actually *do* return bools, but
> this is not reflected in the return type:
>
>
>
> "Using this new mechanism, the is_str_list function in the above example
> would be modified slightly. Its return type would be changed from bool to
> TypeGuard[List[str]]. This promises not merely that the return value is
> boolean, but that a true indicates the input to the function was of the
> specified type.”
>
>
>
> In fact, the promise that it returns a bool is de-facto knowledge you must
> have when you see “TypeGuard” in the return type. It is an implicit
> assumption.
>
>
>
> Generally this might not be a problem, however when a type guard function
> is used for multiple purposes (e.g. a type guard and a “regular” function),
> then the return type is misleading, since a TypeGuard object is *not*
> returned. It’s unclear what type checkers would do in this case.
>
>
>
> The SC debated alternatives, including the decorator syntax specifically
> mentioned in the Rejected Ideas. We also discussed making TypeGuard a
> “wrapping” type defining an __bool__() so that e.g. is_str_list() would be
> defined as such:
>
>
>
> def is_str_list(val: List[object]) -> TypeGuard[List[str]]:
>
> """Determines whether all objects in the list are strings"""
>
> return TypeGuard(all(isinstance(x, str) for x in val))
>
>
>
> but this also isn’t quite accurate, and we were concerned that this might
> be highly inconvenient in practice. In a sense, the type guard-ness of the
> function is an attribute about the function, not about the parameters or
> return type, but there is no way to currently express that using Python or
> type checking syntax.
>
>
>
> I am not sure whether you considered and rejected this option, but if so,
> perhaps you could add some language to the Rejected Ideas about
> it. Ultimately we couldn’t come up with anything better, so we decided
> that the PEP as it stands solves the problem in a practical manner, and
> that this is for the most part a wart that users will just have to learn
> and internalize.
>
>
>
> Cheers,
>
> -Barry (on behalf of the Python Steering Council)
>
>
>
>
>
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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