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No, I'm asking *why* you consider it useful. For example

a) here is this and that problem, and there is absolutely no
way to express this in Python today. If that feature is
added, I can write it in the following way.

b) here is this and that problem. Today, I have to write it
in this way. With the feature added, I can write it in that
way. That way is better than this way because of such and
such reason.

IOW, I would like to see what you are using the default
encoding *for* that makes it desirable to set it through an
environment variable.