Currently many tests in test_curses are skipped if sys.__stdout__ is not attached to terminal. All tests are ran only when run them manually, and without using pager. This leads to passing bugs, like in issue42694.

The proposed PR makes tests always ran. If __stdout__ is not attached to terminal, it tries to attach it to terminal, using __stderr__ if it is attached to terminal, or opening /dev/tty. If neither __stdout__ nor __stderr__ are attached to terminal, it tries to use temporary file, but some functions do not work in this case and will be untested.

It could be better to use os.openpty(), but too many curses outputs can overflow the buffer (2 KiB) and cause the test and all subsequent tests to fail. Currently all tests are passed if use os.openpty(), but I afraid that adding more tests will overflow the buffer and it will confuse future developers. My attempts to solve this issue was unsuccessful for that time, so I left more complicated and less flexible, but more reliable solution.

New changeset 607501abb488fb37e33cf9d35260ab7baefa192f by Serhiy Storchaka in branch 'master':
bpo-42789: Don't skip curses tests on non-tty. (GH-24009)
https://github.com/python/cpython/commit/607501abb488fb37e33cf9d35260ab7baefa192f

Buildbot failure: ARM Raspbian 3.x:
https://buildbot.python.org/all/#/builders/424/builds/658

Tests are run with env var TERM=screen.xterm-256color.

======================================================================
ERROR: test_colors_funcs (test.test_curses.TestCurses)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/lib/buildbot/workers/3.x.gps-raspbian/build/Lib/test/test_curses.py", line 316, in test_colors_funcs
    curses.pair_content(curses.COLOR_PAIRS - 1)
_curses.error: Argument 1 was out of range. (1..COLOR_PAIRS-1)

s390x RHEL8 3.x:
https://buildbot.python.org/all/#/builders/509/builds/601

Tests run with TERM=xterm-256color.

======================================================================
ERROR: test_colors_funcs (test.test_curses.TestCurses)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z/build/Lib/test/test_curses.py", line 316, in test_colors_funcs
    curses.pair_content(curses.COLOR_PAIRS - 1)
_curses.error: Argument 1 was out of range. (1..COLOR_PAIRS-1)

--

test.pythoninfo doesn't log anything about curses. I expected to find _curses.ncurses_version there.

I guess I should merge a PR for issue42681 first.

New changeset 0303008ebceb6ac6035cd9722d1393267304171d by Serhiy Storchaka in branch '3.9':
[3.9] bpo-42789: Don't skip curses tests on non-tty. (GH-24009) (GH-24076)
https://github.com/python/cpython/commit/0303008ebceb6ac6035cd9722d1393267304171d

New changeset 645174abe0d13cce2cb339cc80b095ad484428ea by Serhiy Storchaka in branch '3.8':
[3.8] [3.9] bpo-42789: Don't skip curses tests on non-tty. (GH-24009) (GH-24076) (GH-24078)
https://github.com/python/cpython/commit/645174abe0d13cce2cb339cc80b095ad484428ea

> Buildbot failure: ARM Raspbian 3.x: (...)

As expected, the 3.8 and 3.9 change also broke 11 buildbots (3.8 and 3.8) ;-) I understand that bpo-42681 will fix buildbots.

On other hand, it was impossible to write correct tests for issue42681 while tests were skipped on buildbots. I spent the whole day yesterday debugging and backporting that changes (some tests failed only on one particular builtbot). So it was right to enable tests first.

New changeset b6fc0c406e6b4b86f59b1390142f35da0e364927 by Serhiy Storchaka in branch 'master':
bpo-42789: Enable using /dev/tty in test_curses. (GH-24085)
https://github.com/python/cpython/commit/b6fc0c406e6b4b86f59b1390142f35da0e364927