[issue35123] Add style guide for sentinel usage

[Eric V. Smith]

Eric V. Smith <eric at trueblade.com> added the comment:

Could you give some examples of problems caused by inconsistent usage of sentinels?

I do often wish modules exposed their sentinel values, since it makes writing wrappers easier. It's one of the reasons I ended up exposing dataclasses.MISSING.

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nosy: +eric.smith

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35123>
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