[New-bugs-announce] [issue29757] The loop in utility `socket.create_connection()` swallows previous errors

[Kostis Anagnostopoulos]

New submission from Kostis Anagnostopoulos:

## Context
The utility method `socket.create_connection()` currently works like that:
1. resolve the destination-address into one or more IP(v4 & v6) addresses;
2. loop on each IP address and stop to the 1st one to work;
3. if none works, re-raise the last error.


## The problem
So currently the loop in `socket.create_connection()` ignores all intermediate errors and reports only the last connection failure, 
which  might be irrelevant.  For instance, when both IPv4 & IPv6 networks are supported, usually the last address is a IPv6 address and it frequently fails with an irrelevant error - the actual cause have already been ignored.


## Possible solutions & open questions
To facilitate network debugging, there are at least 3 options:
a. log each failure [as they happen](/python/cpython/blob/6f0eb93183519024cb360162bdd81b9faec97ba6/Lib/socket.py#L717), but that would get the final failure twice: once as a (warning?) message, and once as an exception .
b. collect all failures and log them only when connection fails to collect the errors, 
   but that might miss important infos to the user;
c. collect and return all failures in list attached to the raised exception.

A question for cases (a) & (b) is what logging "means" to use: the `warnings` or `logging` module?
And if `logging` is chosen, log them in `'DEBUG'` or `'WARNING'` level?

Case (c) sidesteps the above questions.

----------
components: Library (Lib)
messages: 289238
nosy: ankostis
priority: normal
pull_requests: 463
severity: normal
status: open
title: The loop in utility `socket.create_connection()` swallows previous errors
versions: Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29757>
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