[typing] PEP 563: Postponed evaluation of annotations: enable it by default in Python 3.11

[vstinner]

BPO	38605
Nosy	@gvanrossum, @warsaw, @brettcannon, @terryjreedy, @ericvsmith, @jwilk, @stevendaprano, @methane, @ambv, @serhiy-storchaka, @vedgar, @ilevkivskyi, @asottile, @pablogsal, @miss-islington, @isidentical, @gousaiyang, @AlexWaygood
PRs	bpo-38605: Make postponed evaluation of annotations default #20434 bpo-38605: bump the magic number for 'annotations' future #22630 bpo-38605: Update "Future statements" docs since PEP 563 is always enabled #25236 bpo-38605: Revert making 'from __future__ import annotations' the default #25490 bpo-38605: Set the release for __future__.annotations to 3.11 #25596 bpo-38605: Update __future__ module doc as annotations is now "mandatory in 3.11" #25602

^{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 = None
created_at = <Date 2019-10-27.16:07:10.414>
labels = ['type-bug', '3.10', 'expert-unicode']
title = '[typing] PEP 563: Postponed evaluation of annotations: enable it by default in Python 3.11'
updated_at = <Date 2022-02-07.11:12:54.549>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2022-02-07.11:12:54.549>
actor = 'jwilk'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Unicode']
creation = <Date 2019-10-27.16:07:10.414>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 38605
keywords = ['patch']
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pr_nums = ['20434', '22630', '25236', '25490', '25596', '25602']
priority = 'normal'
resolution = 'postponed'
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38605'
versions = ['Python 3.10']

[vstinner]

The PEP-563: Postponed evaluation of annotations was introduced an opt-in feature using "from __future__ import annotations". It is scheduled to become the default in Python 4.0.

I would prefer to limit the number of incompatible changes in Python 4.0: it should just a "regular" release, with a regular number of incompatible changes. The version number change is going to cause enough troubles...

Would it be possible possible to enable postponed evaluation of annotations either before or after Python 4.0? For example, can we imagine to enable it by default in Python 3.9? If not, what about Python 3.10?

See also the PEP-608 (Coordinated Python release) and bpo-38604 (Schedule Py_UNICODE API removal).

[ilevkivskyi]

IMO 3.10 would be better, since 3.9 would be too soon (it would be like a schedule for a normal deprecation).

Also if we are really doing this, I think it is better to announce this soon.

Also we should try to fix relevant issues related to string annotations (in typing and dataclasses), like python/typing#508, python/typing#574, https://bugs.python.org/issue37838, https://bugs.python.org/issue34776 and https://bugs.python.org/issue37948.

[gvanrossum]

We never should have mentioned 4.0 as the target date to make this the
default (and only) behavior -- who knows whether there will ever even be a
Python 4.0? Even 3.10 might be on the early side (assuming we'll switch to
a year-long release cycle per PEP-602 -- we will then make deprecations in
general take two release cycles).

I do agree that we should start the process of deprecating the
non-future behavior here in 3.9. I know of one project with a private
fork of Python (for other reasons) that has made this default.

[vstinner]

We never should have mentioned 4.0 as the target date to make this the
default (and only) behavior

I am fine with modifying __future__ documentation to only modify the "Mandatory" column to remove Python 4.0, and then close this issue:
https://docs.python.org/dev/library/__future__.html

--

But I like the "PEP-563: Postponed evaluation of annotations", IMHO it would be nice to get it as the default behavior :-) It's just a matter of properly organize the transition ;-)

[gvanrossum]

You can bring the deprecation schedule up on discourse or python-dev so more folks can let us know whether they'd be okay with 3.9 or 3.10.

[vstinner]

I started a thread on typing-sig:
https://mail.python.org/archives/list/typing-sig@python.org/thread/JG4IWHO3TZORNESR6ARXFL6OUZU2BEBX/

[brettcannon]

I personally like 3.10 as the target as that means users had at least 3 years to move to move over. Plus we can put a warning in the What's New for 3.9 about our plans for 3.10.

[vstinner]

This issue has been discussed during the Language Summit. A quick poll showed that the majority is in favor of changing the default in Python 3.9.

Lukasz proposed a PEP update to propose to switch the default in Python 3.9:
python/peps#1371

For me, the unclear part is which projects would be impacted if the default changes?

Someone mentioned attrs, but it seems like attrs is fine:
python-attrs/attrs#288 (comment)

In term of workflow, I would _prefer_ to get such incompatible in the very beginning of a devcycle, rather than just before the feature freeze. But I don't think that it's a blocker issue. Technically, changes are allowed until 3.9.0 beta1. Moreover, Lukasz is the 3.9 release manager, the author of the PEP-563 and he is in favor of changing the default in 3.9 :-)

[gvanrossum]

Too bad nobody took any action here after the positive outcome of the discussion at the summit.