Consider reintroducing `types.EllipsisType` for the sake of typing #85976

BvB93 · 2020-09-18T19:29:00Z

BPO	41810
Nosy	@gvanrossum, @rhettinger, @ericvsmith, @markshannon, @serhiy-storchaka, @ilevkivskyi, @SamuelMarks, @BvB93, @joshbode
PRs	bpo-41810: Reintroduce `types.EllipsisType`, `.NoneType` & `.NotImplementedType` #22336

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-09-22.15:55:57.057>
created_at = <Date 2020-09-18.19:29:00.494>
labels = ['type-feature', 'library', '3.10']
title = 'Consider reintroducing `types.EllipsisType` for the sake of typing'
updated_at = <Date 2020-11-05.09:51:11.541>
user = 'https://github.com/BvB93'

bugs.python.org fields:

activity = <Date 2020-11-05.09:51:11.541>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2020-09-22.15:55:57.057>
closer = 'gvanrossum'
components = ['Library (Lib)']
creation = <Date 2020-09-18.19:29:00.494>
creator = 'BvB93'
dependencies = []
files = []
hgrepos = []
issue_num = 41810
keywords = ['patch']
message_count = 22.0
messages = ['377135', '377154', '377165', '377167', '377169', '377178', '377248', '377250', '377255', '377259', '377264', '377267', '377269', '377274', '377275', '377277', '377280', '377281', '377282', '377340', '380400', '380401']
nosy_count = 9.0
nosy_names = ['gvanrossum', 'rhettinger', 'eric.smith', 'Mark.Shannon', 'serhiy.storchaka', 'levkivskyi', 'samuelmarks', 'BvB93', 'joshbode']
pr_nums = ['22336']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41810'
versions = ['Python 3.10']

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BvB93 · 2020-09-18T19:29:00Z

Ellipsis is one of the few builtin objects whose type is not exposed via the types module.
This is not really an issue during runtime, as you can always call type(Ellipsis), but for the purpose of typing it is detrimental; the lack of suitable type means that it is impossible to properly annotate a function which takes or returns Ellipsis (unless one is willing to resort to the use of non-public types: python/typeshed#3556).

In order to resolve this issue I propose to reintroduce types.EllipsisType. This should be a fairly simple process, so if there are no objections I'd be willing to give it a shot.

serhiy-storchaka · 2020-09-19T08:57:21Z

Can not type(Ellipsis) be used for typing too?

BvB93 · 2020-09-19T13:32:55Z

If you're asking whether or not one can infer the return type of type(Ellipsis) then yes.
In such case the inferred type is builtins.ellipsis, which is a private stub-only class (see the referenced typeshed issue in my original post).

If you're asking if a valid annotation can be constructed from type(Ellipsis) then the answer is unfortunately no (see below for a few examples).

EllipsisType = type(Ellipsis)

# Both examples are considered invalid
def func1(a: type(Ellipsis): ...
def func2(a: EllipsisType): ...

gvanrossum · 2020-09-19T14:23:06Z

Let’s do this.

serhiy-storchaka · 2020-09-19T15:34:28Z

Would not be better to make MyPy supporting type(Ellipsis)? It would work also on Python versions older than 3.10.

Also, could not Literal[Ellipsis] be used as annotation?

gvanrossum · 2020-09-19T17:12:30Z

Would not be better to make MyPy supporting type(Ellipsis)? It would work also on Python versions older than 3.10.

That would be quite complicated. There is no place in annotations where a function call is currently supported, so I'd prefer not to go there.

Also, could not Literal[Ellipsis] be used as annotation?

Alas, it currently doesn't work (PEP-586 only allows specific types, and type checkers have implemented it exactly). Making it work would be more complicated than the proposal -- once it exists in types.py, it's trivial to add support to mypy.

IMO ideally, eventually, all "hidden" built-in types ought to be exposed somewhere, unless they are truly implementation details -- but since Ellipsis is a first-class singleton object, I don't see how its type could be an implementation detail. (Its name is actually clearly visible in repr(type(Ellipsis)).)

Note that we have started exporting the types of other constructs through types.py, e.g. type(int|str) is types.Union, and type(list[str]) is types.GenericAlias. This is revealed in their repr().

BvB93 · 2020-09-21T10:35:29Z

If we're going ahead with this: PR #22336 contains a concrete implementation of the proposed changes.

markshannon · 2020-09-21T11:10:12Z

I can't resist the pun on typing.
type(...) is the least typing :)

More seriously,

From a general consistency point of view,
if this is to be added, shouldn't types.NoneType and types.NotImplementedType be added as well?

gvanrossum · 2020-09-21T14:27:42Z

I’m okay with adding those (even though NoneType might not work in mypy, given the special-casing for None, per PEP-484).

BvB93 · 2020-09-21T16:17:17Z

Apparently pyright has some interest in NoneType (python/typeshed#4519), so it seems there are already some actual use cases.

In any case, I'm ok with adding NoneType and NotImplementedType to the PR.

gvanrossum · 2020-09-21T17:04:52Z

In any case, I'm ok with adding NoneType and NotImplementedType to the PR.

Please do.

BvB93 · 2020-09-21T18:37:27Z

NoneType and NotImplementedType have been added to the PR as of the latest set of pushes.

ericvsmith · 2020-09-21T19:03:24Z

Does anyone know why types.EllipsisType was removed to begin with? I just want to make sure we're not repeating some mistake of the past.

gvanrossum · 2020-09-21T19:30:15Z

Bas can you do some research to answer Eric's question? In theory it should be possible to use git bisect to find it. I see it in 2.7 but not in 3.4. That's a fairly large interval, alas.

BvB93 · 2020-09-21T19:32:21Z

According to the relevant commit (c9543e4):
"Removed all types from the 'types' module that are easily accessible through builtins."

gvanrossum · 2020-09-21T19:42:13Z

Thanks! Most of the deleted types *are* builtins (e.g. object, int). I assume the reasoning was just that if you wanted the type you could just write type(Ellipsis), type(None) etc.

But that's too dynamic for static checkers, so we want *some* stable name for those back. Do any of the other deleted types strike you as possibly needing to come back?

BvB93 · 2020-09-21T19:59:23Z

Do any of the other deleted types strike you as possibly needing to come back?

I don't think so, no. The only other one that stands out to me is DictProxyType, which has already been reintroduced as MappingProxyType.

ericvsmith · 2020-09-21T20:08:34Z

Thanks for doing the research, Bas! It sounds like adding back in NoneType, NotImplementedType, and EllipsisType is appropriate, then.

+1

The commit should have a comment about the reason: for type checkers which can't use type(Ellipsis), etc. I'll add a comment on the PR about adding a similar note to the blurb.

rhettinger · 2020-09-21T20:57:25Z

Do any of the other deleted types strike you as possibly
needing to come back?

Yes. NoneType would be useful.

gvanrossum · 2020-09-22T15:55:37Z

New changeset 0d0e9fe by Bas van Beek in branch 'master':
bpo-41810: Reintroduce types.EllipsisType, .NoneType & .NotImplementedType (GH-22336)
0d0e9fe

SamuelMarks · 2020-11-05T09:44:15Z

Since we're bringing these back, would you accept a backport of these types?

[I'm writing a bunch of parsers/emitters—and starting to maintain an old runtime type-checker—for 3.6+]

ericvsmith · 2020-11-05T09:51:11Z

I don't think we should backport them. It's definitely a new feature, and our policy is no new features in micro versions.

tsibley · 2023-08-30T20:58:05Z

How does one get access to the type of Ellipsis (for type annotation purposes, not at runtime) on Pythons older than 3.10, i.e. where the types.EllipsisType introduced here isn't available? There's typing_extensions for backported typing features, but no equivalent for types?

(The "right" place to raise this sort of issue/question isn't clear to me, so if this isn't the right place, please let me know or point me elsewhere.)

gvanrossum · 2023-08-30T21:00:04Z

try the typing category on discuss.python.org?

pyright 1.1.325 (released yesterday) added support for @type_check_only decorators¹, which were added (a year ago) to the builtins.ellipsis type of the stdlib typeshed². This type is available only during type-checking and doesn't actually exist at runtime, but previously pyright didn't care about that (and mypy still doesn't). Now it does, noticed thanks to scheduled CI, and throws an error like: "ellipsis" is marked as @type_check_only and can be used only in type annotations (reportGeneralTypeIssues) Explicitly clarify to pyright that our variable is a type alias³ and not a value also used at runtime so it knows its ok to use the type of Ellipsis (via a forward reference to the now-you-see-it, now-you-don't builtins.ellipsis). Relatedly, we can switch from 'builtins.ellipsis' → types.EllipsisType on Python ≥3.10⁴ and stop using the private/internal type for Ellipsis. I'll do that in the subsequent commit. ¹ <microsoft/pyright#5817> ² <python/typeshed@a92da58> <python/typeshed#8531> ³ <https://docs.python.org/3/library/typing.html#typing.TypeAlias> <https://peps.python.org/pep-0613/> ⁴ <https://docs.python.org/3/library/types.html#types.EllipsisType> <python/cpython#85976>

The former is preferable as its not a private/internal type¹, but it's only available on on Python ≥3.10.² ¹ <python/cpython#85976> ² <https://docs.python.org/3/library/types.html#types.EllipsisType>

BvB93 mannequin added 3.10 only security fixes stdlib Python modules in the Lib dir type-feature A feature request or enhancement labels Sep 18, 2020

gvanrossum closed this as completed Sep 22, 2020

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tsibley mentioned this issue Aug 30, 2023

Ellipsis type; fix broken CI nextstrain/cli#306

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Consider reintroducing `types.EllipsisType` for the sake of typing #85976

Consider reintroducing `types.EllipsisType` for the sake of typing #85976

BvB93 mannequin commented Sep 18, 2020

BvB93 mannequin commented Sep 18, 2020

serhiy-storchaka commented Sep 19, 2020

BvB93 mannequin commented Sep 19, 2020

gvanrossum commented Sep 19, 2020

serhiy-storchaka commented Sep 19, 2020

gvanrossum commented Sep 19, 2020

BvB93 mannequin commented Sep 21, 2020

markshannon commented Sep 21, 2020

gvanrossum commented Sep 21, 2020

BvB93 mannequin commented Sep 21, 2020

gvanrossum commented Sep 21, 2020

BvB93 mannequin commented Sep 21, 2020

ericvsmith commented Sep 21, 2020

gvanrossum commented Sep 21, 2020

BvB93 mannequin commented Sep 21, 2020

gvanrossum commented Sep 21, 2020

BvB93 mannequin commented Sep 21, 2020

ericvsmith commented Sep 21, 2020

rhettinger commented Sep 21, 2020

gvanrossum commented Sep 22, 2020

SamuelMarks mannequin commented Nov 5, 2020

ericvsmith commented Nov 5, 2020

tsibley commented Aug 30, 2023

gvanrossum commented Aug 30, 2023

Consider reintroducing types.EllipsisType for the sake of typing #85976

Consider reintroducing types.EllipsisType for the sake of typing #85976

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BvB93 mannequin commented Sep 18, 2020

BvB93 mannequin commented Sep 18, 2020

serhiy-storchaka commented Sep 19, 2020

BvB93 mannequin commented Sep 19, 2020

gvanrossum commented Sep 19, 2020

serhiy-storchaka commented Sep 19, 2020

gvanrossum commented Sep 19, 2020

BvB93 mannequin commented Sep 21, 2020

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rhettinger commented Sep 21, 2020

gvanrossum commented Sep 22, 2020

SamuelMarks mannequin commented Nov 5, 2020

ericvsmith commented Nov 5, 2020

tsibley commented Aug 30, 2023

gvanrossum commented Aug 30, 2023

Consider reintroducing `types.EllipsisType` for the sake of typing #85976

Consider reintroducing `types.EllipsisType` for the sake of typing #85976