PEP 747: Require TypeExpr(X | Y) syntax to spell value of type TypeExpr[X | Y]

[davidfstr]

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[davidfstr]

This PR has been posted for 1 week without comments. Silence is golden?

@JelleZijlstra do you have any comments in advance of merge?

[JelleZijlstra]

I haven't had the time to look in detail at the discussion in the other PR. I would be OK with a less precise specification here, essentially saying that int | str should be assignable to TypeExpr as well as to UnionType and leaving the exact way that happens up to type checkers. For example, type checkers could use a form of type context to achieve this, just like [] right now can be evaluated as either list[int] or list[str] depending on context.

[davidfstr]

I would be OK with a less precise specification here, essentially saying that int | str should be assignable to TypeExpr as well as to UnionType and leaving the exact way that happens up to type checkers.

It sounded like Eric really wanted a precise specification, based on his comment on the previous PR, rather than a less precise one.

To resolve the question of how to recognize T1 | T2, I see 3 general approaches:

1. Define formal rules for the | operator.
   • This is high effort and not something I'm motivated enough to do myself at this time.
2. Use a less precise specification that still allows T1 | T2 to be assignable to TypeExpr.
   • That's the approach of the previous PR.
3. Don't attempt to make T1 | T2 assignable to TypeExpr.
   • That's the approach of this PR.

Right now I'm leaning on approach 3 (i.e. this PR), just to quickly unblock this final question and allow implementation of the PEP in type checkers to proceed. In particular I really want to start implementing the PEP in mypy while I still have time to.