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TextIOWrapper is mostly based on locale.getpreferredencoding(), so msg131290 is still valid: if no env var is set, nl_langinfo() gives 'ASCII' (or something like that). But it is not easy to detect that env vars are not set.
I would prefer a completly different approach: always use UTF-8 by default if the encoding is not set.
Something like:
def open(filename, ..., encoding='UTF-8', ...)
TextIOWrapper.__init__(..., encoding='UTF-8', ...)
So not rely on locales anymore.