[PEP 604] Clarify interaction with ForwardRef, and None
Hi list! I was asked to bring this up here. When PEP604 was first introduced last year I noticed a few under-specified parts, namely how it interacts with ForwardRef and the edge case of `None | None` eg:
from typing import Union Union[int, 'str'] typing.Union[int, ForwardRef('str')] int | 'str' # should this raise an error, or return the same as above? Union[None, "ref1"] typing.Optional[ForwardRef('ref1')] None | "ref1" # should this construct (None first) be forbidden, or does str.__or__ or NoneType.__or__ need to be implemented? Union[None, None] <class 'NoneType'> None | None # should be forbidden? It seems very unlikely to come up in practise even if typing.Union supports it Union["ref1","ref2"] typing.Union[ForwardRef('ref1'), ForwardRef('ref2')] "ref1" | "ref2" # would this be str.__or__ returning types.Union?
In the discussion last year, it was suggested that there could be a `typing.Forward` that could be used instead of the bare string "ref" cases, or even requiring the union to be fully quoted eg `T = "Ref | OtherRef"`.
Thanks for bringing this up! By the time PEP 604 goes live, the final aspect of PEP 563 will also go live: the behavior known as `from __future__ import annotations` will be the default. This means that forward references are only needed rarely (and in some cases for backwards compatibility). So I think it's fine if we don't allow ` int | 'str' `, requiring this to be written instead as ` 'int | str' `. We certainly can't allow ` 'int' | 'str' `. Regarding `None | None`, I think that also shouldn't be allowed -- at least one of the arguments should be a type (or another union, or a generic alias like `list[int]`). I think we will need to add these decisions explicitly to the PEP. Philippe or Maggie, can you propose a way to do this? --Guido On Wed, Jul 29, 2020 at 2:04 PM Richard Eames <github@naddiseo.ca> wrote:
Hi list! I was asked to bring this up here. When PEP604 was first introduced last year I noticed a few under-specified parts, namely how it interacts with ForwardRef and the edge case of `None | None` eg:
from typing import Union Union[int, 'str'] typing.Union[int, ForwardRef('str')] int | 'str' # should this raise an error, or return the same as above? Union[None, "ref1"] typing.Optional[ForwardRef('ref1')] None | "ref1" # should this construct (None first) be forbidden, or does str.__or__ or NoneType.__or__ need to be implemented? Union[None, None] <class 'NoneType'> None | None # should be forbidden? It seems very unlikely to come up in practise even if typing.Union supports it Union["ref1","ref2"] typing.Union[ForwardRef('ref1'), ForwardRef('ref2')] "ref1" | "ref2" # would this be str.__or__ returning types.Union?
In the discussion last year, it was suggested that there could be a `typing.Forward` that could be used instead of the bare string "ref" cases, or even requiring the union to be fully quoted eg `T = "Ref | OtherRef"`. _______________________________________________ Typing-sig mailing list -- typing-sig@python.org To unsubscribe send an email to typing-sig-leave@python.org https://mail.python.org/mailman3/lists/typing-sig.python.org/ Member address: guido@python.org
-- --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/>
participants (2)
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Guido van Rossum -
Richard Eames