Keep deprecated features in Python 3.9 to ease migration from Python 2.7, but remove in Python 3.10

[vstinner]

BPO	39674
Nosy	@gvanrossum, @vstinner, @ezio-melotti, @jamadden, @tirkarthi
PRs	bpo-39674: Revert "bpo-25988: Do not expose abstract collection classes in the collections module. (GH-10596)" #18545 bpo-39674: Suggest to test with DeprecationWarning #18552 bpo-39674: Update collections ABC doc #18747 [3.8] bpo-39674: Update collections ABC deprecation doc #18748 bpo-39674: Revert "bpo-37330: open() no longer accept 'U' in file mode (GH-16959)" #18767 bpo-39674: Fix typo in What's New In Python 3.9 #18776 [3.7] Fix collections ABC deprecation notice #25280 [3.8] bpo-39674: Fix collections ABC deprecation notice #25281

^{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 2020-04-23.23:04:03.240>
created_at = <Date 2020-02-18.11:26:29.040>
labels = ['3.10', 'expert-unicode']
title = 'Keep deprecated features in Python 3.9 to ease migration from Python 2.7, but remove in Python 3.10'
updated_at = <Date 2021-04-08.23:29:27.595>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2021-04-08.23:29:27.595>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-23.23:04:03.240>
closer = 'vstinner'
components = ['Unicode']
creation = <Date 2020-02-18.11:26:29.040>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 39674
keywords = ['patch']
message_count = 13.0
messages = ['362196', '362220', '362222', '362225', '363195', '363263', '363276', '363277', '363308', '363350', '363367', '367159', '390571']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'vstinner', 'ezio.melotti', 'python-dev', 'jmadden', 'xtreak']
pr_nums = ['18545', '18552', '18747', '18748', '18767', '18776', '25280', '25281']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39674'
versions = ['Python 3.10']

[vstinner]

Following discussion on python-dev, I propose to revert the removal of a few deprecated functions to keep them in Python 3.9, and only remove them in Python 3.10. Please see the following email for the longer rationale, and the discussion for further details:

https://mail.python.org/archives/list/python-dev@python.org/thread/EYLXCGGJOUMZSE5X35ILW3UNTJM3MCRE/

With Python 3.8, it was possible to have a single code base working on Python 2.7 and 3.8. Some functions emits DeprecationWarning, but these warnings are ignored (silent) by default. With removed deprecated functions in Python 3.9, *new* code is required to support Python 2.7. The problem is that Python 2.7 is no longer supported. Adding new code to support Python 2.7 sounds painful. Dropping Python 2.7 support isn't free. Projects have to drop Python 2 code, drop CI tests on Python 2, warn users, etc.

The idea is to give maintainers one more year (until Python 3.10) to organize their project to schedule properly the removal of Python 2 support. The first motivation is to ease adoption of Python 3.9.

--

I propose to start with reverting the removal of collections aliases to Abstract Base Classes (ABC) like collections.Mapping alias to collections.abc.Mapping. Removing these aliases is the change which caused most issues when testing Python projects on Python 3.9.

I also propose to modify the What's New In Python 3.9 document to strongly suggest to test your applications with -W default or even -W error to see DeprecationWarning and PendingDeprecationWarning.

[gvanrossum]

Yes please.

I have one exception. I f we manage to get the new parser (pegen) in, we have to remove the old parser module.

[vstinner]

New changeset af5ee3f by Victor Stinner in branch 'master':
bpo-39674: Revert "bpo-25988: Do not expose abstract collection classes in the collections module. (GH-10596)" (GH-18545)
af5ee3f

[vstinner]

PR 18552 adds a section to What's New In Python 3.9 to strongly advice to check for DeprecationWarning in your Python projects.

[vstinner]

Andrew Bartlett of the Samba project asked to revert the removal of the "U" mode:
https://bugs.python.org/issue37330#msg362362

We should consider to revert the removal, and only remove the "U" mode in Python 3.10.

[vstinner]

Another candidate is to revert the ignored "U" mode in open(): commit e471e72 of bpo-37330.

Removing "U" mode of open() broke 11 packages in Fedora:

• aubio
• openvswitch
• python-SALib
• python-altgraph
• python-apsw
• python-magic-wormhole-mailbox-server
• python-munch
• python-parameterized
• python-pylibmc
• python-sphinx-testing
• veusz

Keeping "U" mode in Python 3.9 is also a formal request from Andrew Bartlett of the Samba project: https://bugs.python.org/issue37330#msg362362

[vstinner]

New changeset 91fe414 by Victor Stinner in branch 'master':
bpo-39674: Update collections ABC deprecation doc (GH-18747)
91fe414

[vstinner]

New changeset 2aa694d by Victor Stinner in branch '3.8':
bpo-39674: Update collections ABC deprecation doc (GH-18748)
2aa694d

[vstinner]

New changeset 116fd4a by Victor Stinner in branch 'master':
bpo-39674: Suggest to test with DeprecationWarning (GH-18552)
116fd4a

[vstinner]

New changeset a6d3546 by Victor Stinner in branch 'master':
bpo-39674: Fix typo in What's New In Python 3.9 (GH-18776)
a6d3546

[vstinner]

New changeset 942f7a2 by Victor Stinner in branch 'master':
bpo-39674: Revert "bpo-37330: open() no longer accept 'U' in file mode (GH-16959)" (GH-18767)
942f7a2