gh-157170: Restore use_env() after test_use_prescr_screen in test_curses

[iamsharduld]

test_use_prescr_screen turns use_env off to show that a new_prescr() screen can be used to affect state before initscr(), and never turns it back on. curses.use_env() calls ncurses' use_env(), which sets the process-wide default (_nc_prescreen.use_env) that every later screen copies, not a property of the screen that is current at the time. With it off, ncurses sizes a screen from the terminfo lines/cols capabilities alone, and the linux entry used by the CI runners has none, so every newterm() for the rest of the process returns NULL. That is the 138 errors test_curses reports in the parallel phase of every Ubuntu CI run (all 31 Ubuntu job runs in the last 8 successful runs on main); the sequential rerun passes only because it re-runs the failed tests by name and so never runs ScreenTests first. Details, including an ncurses trace of the failing newterm(), are in the issue.

This restores the setting with addCleanup(). Verified on Ubuntu 25.10 with ncurses 6.5 (the newterm-based setUp() needs ncurses 6.5 or newer):

• TERM=linux ./python -m test test_curses -u curses: 138 errors before, passes after (three consecutive runs).
• TERM=linux ./python -m test --fast-ci test_curses: no rerun needed after.
• TERM=xterm still passes.

On macOS the ScreenTests are skipped (ncurses 6.0), which is why this was invisible on developer machines.

• Issue: test_curses fails in every Ubuntu CI run's parallel phase: test_use_prescr_screen leaves use_env(False) set, so newterm() fails with TERM=linux #157170

[serhiy-storchaka]

Good catch, and thank you for the trace.

Please drop the NEWS entry. This is a fix to unreleased test. It does not need a NEWS entry.

Please simplify the comment, it is too verbose. One addition line would be enough.

[iamsharduld]

Thank you. Both done in a779f46:

• NEWS entry dropped. bedevere/news will stay red until skip news is applied.
• The comment is now a single line: # use_env() is process-wide, not a property of this screen.

The diff is two added lines. Re-ran test_curses with TERM=linux and with TERM=xterm-256color; both pass.

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

: please review the changes made to this pull request.

[serhiy-storchaka]

LGTM. 👍

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Android 3.x (tier-3) has failed when building commit 5f14441.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1591/builds/5200) and take a look at the build logs.
4. Check if the failure is related to this commit (5f14441) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1591/builds/5200

Summary of the results of the build (if available):

Click to see traceback logs

remote: Enumerating objects: 5, done.        
remote: Counting objects:  25% (1/4)        
remote: Counting objects:  50% (2/4)        
remote: Counting objects:  75% (3/4)        
remote: Counting objects: 100% (4/4)        
remote: Counting objects: 100% (4/4), done.        
remote: Compressing objects:  25% (1/4)        
remote: Compressing objects:  50% (2/4)        
remote: Compressing objects:  75% (3/4)        
remote: Compressing objects: 100% (4/4)        
remote: Compressing objects: 100% (4/4), done.        
remote: Total 5 (delta 0), reused 0 (delta 0), pack-reused 1 (from 1)        
From https://github.com/python/cpython
 * branch                    main       -> FETCH_HEAD
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Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at 5f14441114a gh-157170: Restore use_env() after test_use_prescr_screen in test_curses (GH-157171)
Switched to and reset branch 'main'

configure: WARNING: no system libmpdec found; falling back to pure-Python version for the decimal module

../../Parser/lexer/string.c: In function ‘finish_ftstring_expr’:
../../Parser/lexer/string.c:81:20: warning: variable ‘previous_end’ set but not used [-Wunused-but-set-variable]
   81 |         _PyTok_Off previous_end = state->expr_span.start;
      |                    ^~~~~~~~~~~~
In file included from ../../Include/internal/pycore_dict.h:11,
                 from ../../Objects/typeobject.c:8:
In function ‘Py_DECREF_MORTAL’,
    inlined from ‘PyStackRef_CLOSE’ at ../../Include/internal/pycore_stackref.h:703:9,
    inlined from ‘_PyThreadState_PopCStackRef’ at ../../Include/internal/pycore_stackref.h:811:5,
    inlined from ‘vectorcall_maybe’ at ../../Objects/typeobject.c:3118:9:
../../Include/internal/pycore_object.h:448:8: warning: array subscript 0 is outside array bounds of ‘PyObject[0]’ {aka ‘struct _object[]’} [-Warray-bounds]
  448 |     if (--op->ob_refcnt == 0) {
      |        ^

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