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This is a follow-up to PEP 538 that reflects the uncertainty over whether or not the warning on *successful* implicit locale coercion is a good idea.
The argument for this warning is that it alerts redistributors, system integrators, and application developers to the fact that LC_CTYPE may not be what they expect it to be.
The argument against it is that in many, and potentially even most, cases where the warning is omitted, there won't be any subsequent problems, and so the warning qualifies as a false alarm (especially for end users of applications that just happen to be written in Python), and the PEP 538 section in the .37 What's New, together with the fact that "LC_CTYPE=C.UTF-8" (or similar) appears in the process environment can be considered sufficient notice of the change for debugging purposes.
The initial PEP 538 implementation at https://github.com/python/cpython/pull/659 includes the warning, this issue reflects the possibility that we may decide to reverse that decision prior to the release of Python 3.7.0.

My latest proposal on python-dev:
- no warning by default on successful coercion
- set "PYTHONCOERCECLOCALE=warn" to enable the warning when it's considered a configuration error in the runtime environment

Some relevant mailing list threads on the usability problems posed by issuing the warning by default:
python-dev (impact on developer experience): https://mail.python.org/pipermail/python-dev/2017-June/148323.html
Fedora's python-devel (build environment warnings due to F26 backport of PEP 538): https://lists.fedorahosted.org/archives/list/python-devel@lists.fedoraproject.org/thread/VSEGOF76XMBJOAO4C2MORNK3I2VIPOTU/ 

The warnings caused something around 27 test failures on "AMD64 FreeBSD CURRENT Non-Debug 3.x" buildbot:
http://buildbot.python.org/all/builders/AMD64%20FreeBSD%20CURRENT%20Non-Debug%203.x/builds/438/steps/test/logs/stdio
27 tests failed again:
 test_asyncio test_base64 test_c_locale_coercion test_capi
 test_cmd_line test_cmd_line_script test_compileall
 test_concurrent_futures test_doctest test_faulthandler
 test_file_eintr test_inspect test_io test_json test_keyword
 test_module test_multiprocessing_fork
 test_multiprocessing_forkserver test_multiprocessing_main_handling
 test_subprocess test_symbol test_sys test_threading test_tools
 test_traceback test_venv test_warnings

I wrote a PR to remove PEP 538 warnings:
https://github.com/python/cpython/pull/2186
Reference (unpatched):
haypo@selma$ env -i LC_ALL=C ./python -c "import locale; print(locale.getpreferredencoding())"
Python runtime initialized with LC_CTYPE=C (a locale with default ASCII encoding), which may cause Unicode compatibility problems. Using C.UTF-8, C.utf8, or UTF-8 (if available) as alternative Unicode-compatible locales is recommended.
ANSI_X3.4-1968
haypo@selma$ env -i LC_CTYPE=C ./python -c "import locale; print(locale.getpreferredencoding())"
Python detected LC_CTYPE=C: LC_CTYPE coerced to C.UTF-8 (set another locale or PYTHONCOERCECLOCALE=0 to disable this locale coercion behavior).
UTF-8
haypo@selma$ env -i LC_ALL=C ./python -c "import locale; print(locale.getpreferredencoding())"
Python runtime initialized with LC_CTYPE=C (a locale with default ASCII encoding), which may cause Unicode compatibility problems. Using C.UTF-8, C.utf8, or UTF-8 (if available) as alternative Unicode-compatible locales is recommended.
ANSI_X3.4-1968
With my change:
haypo@selma$ env -i ./python -c "import locale; print(locale.getpreferredencoding())"
UTF-8
haypo@selma$ env -i LC_CTYPE=C ./python -c "import locale; print(locale.getpreferredencoding())"
UTF-8
haypo@selma$ env -i LC_ALL=C ./python -c "import locale; print(locale.getpreferredencoding())"
ANSI_X3.4-1968

As Victor notes above, for systems where no suitable coercion target locale is available, even the "unsupported locale" warning is an issue, since it's only full Unicode text handling that's unsupported in such locales - you can still process ASCII-only and non-text data just fine in such environments (and even Unicode data if you're sufficiently careful).
As a matter of future-proofing CPython, we also need to account for the fact that some C implementations (potentially including future versions of glibc) may starting using UTF-8 as the default text encoding in the C locale itself (requiring the use of variants like C.ASCII or C.latin-1 to opt-in to the use of legacy text encodings).
I still think the warnings are potentially valuable for system integrators and operating system developers trying to ensure that C.UTF-8 is genuinely pervasive as their default locale, so my counter-proposal to Victor's PR is:
* remove the build-time warning flag
* gate the warnings for both successful and unsuccessful locale coercion behind a runtime check for PYTHONCOERCECLOCALE=warn
That means:
* we provide the best possible developer experience we can by default (i.e. no visible warnings, we assume UTF-8 where possible)
* redistributors have a relatively simple patch to always emit the warnings if they want to do so (i.e. remove the conditional checks)
* system integrators and folks debugging application misbehaviour can readily enable the warnings at runtime as part of fault investigation activities

Updated issue title to reflect the fact we're now considering just silencing *all* the warnings by default.

If this warnings are disabled by default, who enable it?
How about just remove them?
I'm OK to remove them all.
Since it's not ideal, nothing go worse than Python 3.6.
Additionally, if PEP 540 is accepted, we can use UTF-8 for
stdio and filesystem encoding even when there are no UTF-8 locale.

By having the warnings always compiled in, but off by default, "PYTHONCOERCECLOCALE=warn" becomes a debugging tool for integration failures that we (or end users) suspect might be due to the locale coercion behaviour. It's essentially an even more esoteric variant of tools like "PYTHONINSPECT=1", "python -X faulthandler", "python -v", etc.
I already learned something myself based on updating the test cases to cover the opt-in warning model: I initially thought that when we set "LC_ALL=C" we'd get *both* warnings, but we don't (at least with glibc).
My working theory is that setting 'LC_ALL=C' causes 'setlocale(LC_CTYPE, "C.UTF-8")' to fail, even if there's otherwise a C.UTF-8 locale available. (I haven't conclusively proven that theory, but it's the most likely point for the locale coercion to be implicitly bypassed)

As I wrote on python-dev, I would prefer no warning and no option to enable
warnings. But it's not my PEP, I would prefer that Nick makes a choice
here. Right now, my main worry is that my little (freebsd) buildbots are
still sick

OK, based on the latest round of custom buildbot results (e.g. http://buildbot.python.org/all/builders/AMD64%20FreeBSD%20CURRENT%20Debug%20custom/builds/12/steps/test/logs/stdio ), it looks like the main remaining problems are those covered by issue 30672, where *BSD platforms handle some locales differently from the way Linux handles them (and Mac OS X then inherits those differences), and issue 30647 (where attempting to use the UTF-8 locale can cause nl_langinfo to fail).
So for the latest iteration on *this* change, I'm going to do the following:
1. Disable "UTF-8" as a candidate target locale
2. Adjust the test suite's expectations accordingly

New changeset eb81795d7d3a8c898fa89a376d63fc3bbfb9a081 by Nick Coghlan in branch 'master':
bpo-30565: Add PYTHONCOERCECLOCALE=warn runtime flag (GH-2260)
https://github.com/python/cpython/commit/eb81795d7d3a8c898fa89a376d63fc3bbfb9a081

OK, PEP 538 is effectively disabled on FreeBSD and Mac OS X now, and the locale coercion and compatibility warnings are off by default everywhere.
PYTHONCOERCECLOCALE=warn is explicitly documented as a debugging tool for use when folks already suspect that locale coercion or legacy locale incompatibility might be to blame for particular problems that they're seeing.