gh-148603: Update docs now UTF-8 is default (PEP 686)

[hugovk]

Fixes #148603.

Also update headers to sentence case in modified files:

https://devguide.python.org/documentation/style-guide/#capitalization

• Issue: open() documentation still says default encoding is locale-dependent in 3.15 #148603

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #34536367 | 📁 Comparing 0823961 against main (575fe39)

  🔍 Preview build  

40 files changed · + 8 added · ± 31 modified · - 1 deleted

+ Added

• builtins/constants.html
• builtins/exceptions.html
• builtins/functions.html
• builtins/index.html
• builtins/stdtypes.html
• builtins/threadsafety.html
• builtins/time-complexity.html
• library/tkinter.dialogs.html

± Modified

• glossary.html
• index.html
• c-api/bytes.html
• extending/index.html
• faq/general.html
• library/builtins.html
• library/constants.html
• library/csv.html
• library/exceptions.html
• library/functions.html
• and 21 more...

- Deleted

• library/dialog.html

[StanFromIreland]

This reads oddly.

[hugovk]

I've updated it, is it better now?

[vstinner]

LGTM. The changes LGTM, I didn't check if the change is exhaustive (update all references to open() default encoding).

I have a question for @methane about EncodingWarning, but it should not block this PR.

[vstinner]

@methane: It now looks wrong to emit EncodingWarning if open() is called with no encoding argument, since it defaults to UTF-8 which is fine, no? For me, the purpose of PEP 597 "Add optional EncodingWarning" is to detect when platform dependent locale encoding is used. Am I wrong?

At least, I see one advantage of the current behavior, it helps writing new code which is not platform dependent when running on older Python versions.

[methane]

EncodingWarning is a tool for people who want to identify all the places that will be affected by the transition to UTF-8 Mode and verify that the transition will not cause any problems.

Considering that some people may not start thinking about enabling UTF-8 Mode until Python 3.15, I think removing EncodingWarning immediately in Python 3.15 would be premature. How about removing it in Python 3.17 instead?

[vstinner]

Since -X warn_default_encoding is an opt-in option to emit EncodingWarning, I'm fine with emitting EncodingWarning on Python 3.15 when the encoding parameter is omitted. As I wrote, it should help writing new code on Python 3.15 which is compatible (behave as expected) on older Python versions.

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-157556 is a backport of this pull request to the 3.15 branch.