PYTHONCOERCECLOCALE=1 should not force C locale coercion, Python should still check if the LC_CTYPE locale is "C". Bug: $ ./python -c 'import locale; print(locale.setlocale(locale.LC_CTYPE, None))' fr_FR.UTF-8 $ PYTHONCOERCECLOCALE=1 ./python -c 'import locale; print(locale.setlocale(locale.LC_CTYPE, None))' C.UTF-8 It should be fr_FR.UTF-8 as well in the second example :-( It seems to be a regression that I introduced in the middle of my refactoring on the Python initialization. The bug is around: static void config_init_locale(_PyCoreConfig *config) { if (config->coerce_c_locale < 0) { /* The C locale enables the C locale coercion (PEP 538) */ if (_Py_LegacyLocaleDetected()) { config->coerce_c_locale = 1; } } ... } The 3.7 branch and the 3.7.0 release are affected :-( $ ./python -V Python 3.7.0 $ PYTHONCOERCECLOCALE=1 ./python -c 'import locale; print(locale.setlocale(locale.LC_CTYPE, None))' C.UTF-8

 New changeset 55e498058faf8c97840556f6d791c2c392732dc3 by Victor Stinner in branch 'master': bpo-35336: Fix PYTHONCOERCECLOCALE=1 (GH-10806) https://github.com/python/cpython/commit/55e498058faf8c97840556f6d791c2c392732dc3 

> PYTHONCOERCECLOCALE=1 should not force C locale coercion, Python should still check if the LC_CTYPE locale is "C". Reference: https://docs.python.org/dev/using/cmdline.html#envvar-PYTHONCOERCECLOCALE "If (...) the current locale reported for the LC_CTYPE category is either the default C locale, or else the explicitly ASCII-based POSIX locale, (...)"

 New changeset df738d56fe798b3586ed71775df25bf127789cf6 by Victor Stinner in branch '3.7': bpo-35336: Fix PYTHONCOERCECLOCALE=1 (GH-10806) (GH-10813) https://github.com/python/cpython/commit/df738d56fe798b3586ed71775df25bf127789cf6