Am 21.09.20 um 16:28 schrieb Zac Hatfield Dodds:
> Rather than defining a new Any type, this seems to me rather like a
> TypeVar with what the Mypy docs call value restriction (not a bound). 
> For example:
>
> > * ip_address() can return either an IPv4Address or an IPv6Address,
> depending on the input string.
> > * urlopen() can return either an HTTPResponse or an addinfourl.
>
> def ip_address(s: str) -> TypeVar("address", IPv4Address,
> IPv6Address): ...
> # and similarly for urlopen()
>
> I think this would also require new logic from typecheckers because I
> don't believe you can currently have a return typevar without an
> argument typevar, but the spelling makes sense to me and narrowing
> should be as obvious as it ever is between typecheckers.
>
I don't think that TypeVars are a good fit. As you point out, TypeVars
only make sense if there are at least two instances of it in a
definition. They have a completely different purpose (being variables)
than what AnyOf is supposed to do.

> > * json.loads() et al. can return a selection of types, depending on
> the input.
>
> JSON is really a case for recursive types - "we usually omit the base
> case" isn't much worse to me than "one or more layers in it's just Any".

It would definitely benefit greatly from recursive types, although I
think this is a bit of an orthogonal issue.

  - Sebastian
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