Add option to get all errors from create_connection in an ExceptionGroup

[bitdancer]

BPO	29980
Nosy	@gvanrossum, @terryjreedy, @tiran, @bitdancer, @1st1, @nanjekyejoannah, @iritkatriel

^{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 2017-04-04.17:12:21.325>
labels = ['expert-asyncio']
title = 'OSError: multiple exceptions should preserve the exception type if it is common'
updated_at = <Date 2020-11-25.23:31:25.518>
user = 'https://github.com/bitdancer'

bugs.python.org fields:

activity = <Date 2020-11-25.23:31:25.518>
actor = 'christian.heimes'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2017-04-04.17:12:21.325>
creator = 'r.david.murray'
dependencies = []
files = []
hgrepos = []
issue_num = 29980
keywords = []
message_count = 5.0
messages = ['291126', '291127', '291288', '340147', '381869']
nosy_count = 8.0
nosy_names = ['gvanrossum', 'terry.reedy', 'christian.heimes', 'r.david.murray', 'aymeric.augustin', 'yselivanov', 'nanjekyejoannah', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue29980'
versions = []

[bitdancer]

create_connection will try multiple times to connect if there are multiple addresses returned by getaddrinfo. If all connections fail it inspects the exceptions, and raises the first one if they are all equal. But since the addresses are often different (else why would we try multiple times?), the messages will usually be different. When the messages are different, the code raises an OSError with a list of the exceptions so the user can see them all. This, however, loses the information as to *what* kind of exception occurred (ie: ConnectioRefusedError, etc).

I propose that if all of the exceptions raised are of the same subclass, that that subclass be raised with the multi-message list, rather than the base OSError.

[bitdancer]

If all connections fail (not file :)

[terryjreedy]

I agree. And if the sub-exceptions are different, prepend them to the messages in the OSError list.

[aymericaugustin]

A very similar issue came up here: python-websockets/websockets#593

When raising an exception that gathers multiple sub-exceptions, it would be nice to be able to iterate the exception to access the sub-exceptions.

[tiran]

The feature request reminds me of python-trio/trio#611. Nathaniel and Yury have been discussing the idea of a MultiError handler for a while.

[iritkatriel]

Do we want to do this with ExceptionGroup for 3.11?

[gvanrossum]

Perhaps, though we’d have to add a flag to enable that behavior, since raising EG would be a backwards incompatible change.