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To get the new PEP 538 tests passing on Mac OS X (see [1,2]), I ended up having to skip the following test scenarios:
 LANG=UTF-8 (behaves like LANG=C, *not* LC_CTYPE=UTF-8)
 LANG=POSIX (behaves like a distinct locale is set, not LANG=C)
 LC_CTYPE=POSIX (behaves like a distinct locale is set, not LANG=C)
 LC_ALL=POSIX (behaves like a distinct locale is set, not LANG=C)
However, I'm not sure whether that should be diagnosed as a pure testing problem, where we change the test's expectations to match the current behaviour, or a bug in the PEP 538 implementation, where we should be updating it to produce the behaviour that the tests were originally expecting.
[1] https://github.com/python/cpython/commit/4563099d28e832aed22b85ce7e2a92236df03847
[2] https://github.com/python/cpython/commit/7926516ff95ed9c8345ed4c4c4910f44ffbd5949 )

For the case where POSIX is a distinct locale from the default C locale (rather than a simple alias), I'm leaning towards taking PEP 538 literally, and adjusting the affected test cases to assume that locale coercion *won't* happen for that case on Mac OS X.
I *think* we can check for the alias behaviourally by setting the "POSIX" locale and seeing if it reports itself back to us as "C", but if not, then I'll just add in a sys.platform check, and we can figure out a behavioural check later if the platform check turns out to cause problems.

It seems the "POSIX is not just an alias for the C locale" behaviour is inherited from *BSD, rather than being specific to Mac OS X: http://buildbot.python.org/all/builders/AMD64%20FreeBSD%20CURRENT%20Debug%20custom/builds/12/steps/test/logs/stdio 

Flagging issue 30647 as a dependency, since the problem with breaking nl_langinfo will need to be fixed before these tests can be re-enabled.

As discussed in https://bugs.python.org/issue32238 and https://mail.python.org/pipermail/python-dev/2017-December/151105.html, I now think the right answer for the POSIX case is to ensure the legacy locale detection logic always treats that the same way as it does the C locale.
LANG=UTF-8 I'm still not sure about - as I understand it, that's a partial locale that only defines LC_CTYPE, which may be why the libc implementation ignores it if you try to set it as a general category default.

Ned, I'd forgotten about the part of this issue which amounts to "Check for 'POSIX' as a locale coercion trigger in addition to 'C', as not every platform aliases the former to the latter the way glibc does".
So while I don't think this is really a release blocker (as you have to explicitly request the POSIX locale to encounter the discrepancy, and I think fixing it in 3.7.1 would be OK), I *am* bumping it up to "critical", as I think it would be better to resolve this for 3.7.0b4 rather than letting it linger further.

Thanks for the headsup, Nick. BTW, some of the discussion in languishing Issue18378 might be relevant here.

The locale module has its own extra layer of oddities that I don't personally understand - #29571 and #20087 are another couple of issues along those lines.

Putting back to normal, as the difference between the C locale and the POSIX locale is that you never get the latter by default - you have to explicitly request it.
The underlying fix for this is in the PR for bpo-34589.

Since this issue has been created, I deeply reworked the Python Initialization with the PEP 587, and I made many changes related to locales and the UTF-8 Mode (PEP 540). What's the status of this issue?

There are a couple of cases that the C locale coercion tests skip because I don't (or didn't) know what they *should* do:
* https://github.com/python/cpython/blob/24dc2f8c56697f9ee51a4887cf0814b6600c1815/Lib/test/test_c_locale_coercion.py#L262 (skips the "LANG=UTF-8" test)
* https://github.com/python/cpython/blob/24dc2f8c56697f9ee51a4887cf0814b6600c1815/Lib/test/test_c_locale_coercion.py#L37 (only adds "POSIX" to the expected C locale equivalents list on non-Android Linux systems)
With the interpreter explicitly checking for "POSIX" now, at least the latter special case could potentially be removed, with "POSIX" just always being part of the EXPECTED_C_LOCALE_EQUIVALENTS list. (It was only removed because it used to break on FreeBSD and Mac OS X)
I don't think we ever figured out why the "LANG=UTF-8" case didn't work the way we expected, so I suspect where going to need to keep that skip for now.

(Removed the patch keyword, as the linked PR was for an old change that didn't cover the remaining test issues)

Removing issue assignment, as I'm not actively investigating this.