While Victor and I reconciled the respective designs & implementations of PEP 538/540 (with 538 relying on 540 to handle the "No suitable target locale" case, and 540 relying on 538 to handle extension modules like GNU readline), the interaction between the two mechanisms currently isn't clear in the documentation.

So, before 3.7rc1 goes out, we should:

1. Combine the presentation of the two PEPs in the What's New documentation to make it clearer that they're tackling different aspects of the same problem
2. Clarify in the reference docs for PYTHONCOERCECLOCALE and PYTHONUTF8MODE that they're independent settings (so you have to explicitly turn them *both* off if you truly want to force ASCII based text handling in the C or POSIX locale)



(Issue raised based on a Twitter discussion with Anthony Shaw at https://twitter.com/anthonypjshaw/status/991614899791933441)

Ned, just adding you to the nosy list here, as I don't think this is a release blocker per se (since nothing's actually *broken*), but the current state of the PEP 538/540 docs is such that it isn't going to be clear to most readers why both mechanisms exist, or how they interact with each other.

Nick, thanks for the notification.  I assume this means you are planning to provide one or more updates.  If so, any idea when?

I'm aiming for this week, so with the docstring-AST-induced delay I should even meet the original rc1 goal :)

New changeset 1bcb8a636857e3383d65aaf196f93edb949f2e79 by Nick Coghlan in branch 'master':
bpo-33409: Clarify PEP 538/540 relationship (GH-7534)
https://github.com/python/cpython/commit/1bcb8a636857e3383d65aaf196f93edb949f2e79

New changeset dd613cf335d7556e20ef3dd01c3abc081b958449 by Miss Islington (bot) in branch '3.7':
bpo-33409: Clarify PEP 538/540 relationship (GH-7534)
https://github.com/python/cpython/commit/dd613cf335d7556e20ef3dd01c3abc081b958449