I have a system where the default locale is C.UTF-8, and en_US.UTF-8 is
not installed.

But locale.normalize() unhelpfully converts "C.UTF-8" to "en_US.UTF-8".

So the following crashes for me:

  python3.6 -c "import locale;locale.setlocale(locale.LC_ALL, ('C', 'UTF-8'))"


Similarly getdefaultlocale() returns ('en_US', 'UTF-8'), so this crashes too:

  export LANG=C.UTF-8
  unset LC_CTYPE
  unset LC_ALL
  unset LANGUAGE
  python3.6 -c "import locale;locale.setlocale(locale.LC_ALL, locale.getdefaultlocale())"


This behaviour is caused by a locale_alias entry in Lib/locale.py .

https://bugs.python.org/issue20076 documents its addition but doesn't
provide a rationale.

I can see that it might be helpful to provide such a conversion if
C.UTF-8 doesn't exist and en_US.UTF-8 does, but the current code is
breaking modern correctly-configured systems for the benefit of old
misconfigured ones (C.UTF-8 shouldn't really be in the environment if it
isn't available on the system, after all).