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.