ContextManager and AsyncContextManager protocols can't be subclassed

[jivid]

BPO	38008
Nosy	@ilevkivskyi, @miss-islington, @jivid
PRs	bpo-38008: Move builtin protocol whitelist to mapping instead of list #15647 [3.8] bpo-38008: Move builtin protocol whitelist to mapping instead of list (GH-15647) #16021

^{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 2019-09-12.11:59:49.588>
created_at = <Date 2019-09-02.11:44:10.836>
labels = ['3.8', 'type-bug', 'library', '3.9']
title = "ContextManager and AsyncContextManager protocols can't be subclassed"
updated_at = <Date 2019-09-12.11:59:49.587>
user = 'https://github.com/jivid'

bugs.python.org fields:

activity = <Date 2019-09-12.11:59:49.587>
actor = 'levkivskyi'
assignee = 'none'
closed = True
closed_date = <Date 2019-09-12.11:59:49.588>
closer = 'levkivskyi'
components = ['Library (Lib)']
creation = <Date 2019-09-02.11:44:10.836>
creator = 'divijrajkumar'
dependencies = []
files = []
hgrepos = []
issue_num = 38008
keywords = ['patch']
message_count = 5.0
messages = ['350988', '350989', '351000', '352088', '352096']
nosy_count = 3.0
nosy_names = ['levkivskyi', 'miss-islington', 'divijrajkumar']
pr_nums = ['15647', '16021']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38008'
versions = ['Python 3.8', 'Python 3.9']

[jivid]

Reported this initially on github - python/typing#650

The problem is in the typing code that checks if any of the parent classes for a Protocol are builtin protocols. The code today maintains a whitelist of builtin protocols that are okay to be subclassed, but expects them all to live within the 'collections.abc' module. This means 'typing.ContextManager' and 'typing.AsyncContextManager' cannot be subclassed, even though those protocols are listing in the whitelist.

[jivid]

I'd like to take a stab at putting up a patch for this but I've never contributed anything to Python before, so I'll have a read through the contribution docs and put up a PR.

[ilevkivskyi]

I'd like to take a stab at putting up a patch for this

Great, thanks! Go ahead and try it.

[ilevkivskyi]

New changeset 692a0dc by Ivan Levkivskyi (Divij Rajkumar) in branch 'master':
bpo-38008: Move builtin protocol whitelist to mapping instead of list (GH-15647)
692a0dc

[miss-islington]

New changeset 52baf90 by Miss Islington (bot) in branch '3.8':
bpo-38008: Move builtin protocol whitelist to mapping instead of list (GH-15647)
52baf90