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I've recently come across a strange failure in the tests for the input()
built-in function:
 $ ./python -E -m test -v test_readline test_builtin
 [... snip ...]
 ======================================================================
 FAIL: test_input_tty_non_ascii (test.test_builtin.BuiltinTest)
 ----------------------------------------------------------------------
 Traceback (most recent call last):
 File "/home/nadeem/src/cpython/def/Lib/test/test_builtin.py", line 1079, in test_input_tty_non_ascii
 self.check_input_tty("prompté", b"quux\xe9", "utf-8")
 File "/home/nadeem/src/cpython/def/Lib/test/test_builtin.py", line 1070, in check_input_tty
 self.assertEqual(input_result, expected)
 AssertionError: 'quux' != 'quux\udce9'
 - quux
 + quux\udce9
 ? +
 ======================================================================
 FAIL: test_input_tty_non_ascii_unicode_errors (test.test_builtin.BuiltinTest)
 ----------------------------------------------------------------------
 Traceback (most recent call last):
 File "/home/nadeem/src/cpython/def/Lib/test/test_builtin.py", line 1083, in test_input_tty_non_ascii_unicode_errors
 self.check_input_tty("prompté", b"quux\xe9", "ascii")
 File "/home/nadeem/src/cpython/def/Lib/test/test_builtin.py", line 1070, in check_input_tty
 self.assertEqual(input_result, expected)
 AssertionError: 'quux' != 'quux\udce9'
 - quux
 + quux\udce9
 ? +
The failure only manifests itself if the readline module is loaded before
test_builtin runs (hence the presence of test_readline above). It will
not occur if regrtest is run with either of the -j or -W flags (which is
why it hasn't been seen on the buildbots).
The problem seems to be that readline assumes that its input should use
the locale encoding, and silently strips out any undecodable chars. This
breaks the tests mentioned above, since they set up sys.stdin to use the
surrogateescape error handler, expecting invalid characters to be escaped
rather than discarded.
This problem doesn't crop up if readline is *not* loaded, because in that
case PyOS_Readline() falls back to a stdio-based implementation
(PyOS_StdioReadline()) that preserves invalid characters, allowing them
to be handled properly by sys.stdin's encoding and error handler.
I have been able to fix the test failures with the attached patch, which
stops readline from eating invalid characters, making it consistent with
the stdio-based fallback. Can someone with more knowledge of readline
and/or locale issues advise whether the change is a good idea?

Here's another patch that ensures the test always exercises the GNU
readline code path (rather than the stdio fallback). This will cause the
failure to occur when running just test_builtin (no need to also run
test_readline before it).
Ideally we'd want to test both code paths, but I'm not sure how to
accomplish that reliably, short of running the test in a subprocess.

I had exactly the same error on 3.3b1 when running the test suite with randomized order.

This problem still exists on 3.4.
The attached patch seems to solve it.

New changeset cf0f450b3299 by Nadeem Vawda in branch '3.2':
Issue #13886: Fix input() to not strip out supposedly-invalid input bytes.
http://hg.python.org/cpython/rev/cf0f450b3299 

New changeset 5c8732049dd5 by Nadeem Vawda in branch '3.3':
Issue #13886: Fix input() to not strip out supposedly-invalid input bytes.
http://hg.python.org/cpython/rev/5c8732049dd5 

New changeset 9774721bfc32 by Nadeem Vawda in branch 'default':
Issue #13886: Fix input() to not strip out supposedly-invalid input bytes.
http://hg.python.org/cpython/rev/9774721bfc32 

New changeset 12223782031f by Nadeem Vawda in branch '2.7':
Issue #13886: Fix input() to not strip out supposedly-invalid input bytes.
http://hg.python.org/cpython/rev/12223782031f 

- setlocale(LC_CTYPE, "");
+ setlocale(LC_CTYPE, "C");
This looks dangerous to me. Are you sure readline's behavior doesn't change because of this?

You're right; it breaks backspacing over multibyte characters. I should
have tested it more carefully before committing. I'll revert the changes.

New changeset e6952acd5a55 by Nadeem Vawda in branch '3.2':
Back out fix for issue #13886; it introduced a new bug in interactive readline use.
http://hg.python.org/cpython/rev/e6952acd5a55 

New changeset 5c7e884b205a by Nadeem Vawda in branch '3.3':
Back out fix for issue #13886; it introduced a new bug in interactive readline use.
http://hg.python.org/cpython/rev/5c7e884b205a 

New changeset 8b8c6abda7e8 by Nadeem Vawda in branch 'default':
Back out fix for issue #13886; it introduced a new bug in interactive readline use.
http://hg.python.org/cpython/rev/8b8c6abda7e8 

New changeset 5bf91dfb1e34 by Nadeem Vawda in branch '2.7':
Back out fix for issue #13886; it introduced a new bug in interactive readline use.
http://hg.python.org/cpython/rev/5bf91dfb1e34 

Issue #18230 is another test_builtin failure related to tty tests.

This happens even when run test_builtin twice.
 ./python -E -m test -v test_builtin test_builtin
or
 ./python -E -m test -Fv test_builtin

I don’t think this affects Python 2. The failing tests were added in revision 421c8e291221, Issue 13342, for 3.2+ only. They invole input() doing text decoding. AFAIK Python 2’s equivalent, raw_input(), does not do text decoding.
I suspect we can’t really change how Readline handles text encoding errors, which seems to be what Nadeem was trying to do. I suggest to just fix the tests without changing Readline.
As far as I know there is no way to un-register the Readline module once it has been loaded. A quick and dirty workaround might be to skip the test(s) if the Readline has been loaded ("readline" in sys.modules). But a better fix would probably be to run the test in a subprocess, where we can start a new interpreter from scratch and control whether Readline is loaded.
Looking closer at the tests, they mention invoking Gnu Readline. But the associated bug fix is in the wrapper code around PyOS_Readline(), which may call Gnu Readline if it is loaded, or may call a simpler internal routine otherwise. So ideally the tests should be repeated with Readline unloaded and loaded.
Also, the comment for test_input_tty_non_ascii() implies it is testing UTF-8 encoding. But the data is not valid UTF-8 so it ends up testing the error handling. I think that test should use valid UTF-8 input.

Here is a possible patch which:
* Runs the three test_input_tty() tests in a fresh child Python interpreter, for control over whether Readline is loaded. A pseudoterminal is used for stdin and stdout, and stderr is left untouched to easily see error messages.
* Repeats each test with and without Readline enabled
* Only tests the error handling without Readline
* Fixes the non-error test to use proper UTF-8 input
With my patch applied, there are a couple of prompts mixed up in the test log output via stderr, due to Issue 1927. It is not a major problem, but perhaps we should work on fixing that bug first.

Since people keep coming upon this bug, perhaps we should inhibit push my fix without fixing that other prompt bug (now a feature change I think). Probably have to capture stderr to avoid it coming out in the test output.

input-readline.v2.patch sets stderr=DEVNULL so that the prompt does not come out in the test log.
A disadvantage of this is that if there is a failure, any error messages, stack trace, etc is also lost. To fix this properly, we would probably have to capture stderr using a generalized communicate() loop, like I have done at the bottom of <https://hg.python.org/cpython/annotate/v3.6.0b4/Lib/test/test_readline.py#l231> (see also Issue 1260171).

Hi Xavier, I was about to push input-readline.v2.patch, but I thought it might be good to check with you first if this causes problems with Android, based on my experience with Issue 28997. With the patch applied, test.test_builtin.PtyTests.test_input_tty_non_ascii() will call the input() function twice, with and without Readline enabled. It inputs non-ASCII data in into a pseudoterminal, and expects to see it in the function return value. Perhaps we may need to skip the Readline half of the test on Android.

Thanks for waiting for a run of the patch on Android.
Indeed, test_input_tty_non_ascii fails with this patch on Android when LANG is not set and is ok when LANG=en_US.UTF-8. As you are suggesting, the test is ok when skipping the Readline half of the test. This may be done by adding 'is_android' in the 'from test.support import ...' statement and updating the test with:
 if readline and not is_android:
 tests.append(True)

V3 of my patch skips the Readline tests in cases involving non-ASCII bytes when the locale seen by Readline would be ASCII. Readline may translate the non-ASCII bytes to escape sequences, and treat them as special Meta (Alt) key combinations. This behaviour depends on Readline configuration ("set convert-meta off" in /etc/inputrc in my case).
It also includes a potential workaround for Android, depending on the resolution of Issue 28997.

With input-readline.v3.patch, test_builtin runs with success on Android api 21.
With pep538_coerce_legacy_c_locale_v4.diff that implements PEP 538 in issue 28180, and with input-readline.v3.patch modified to have 'readline_encoding = locale.getpreferredencoding()' even when 'is_android' is True, test_builtin runs with success. This means that no specific Android handling would be needed if PEP 538 were to be adopted.
The new input-readline.v4.patch is a slight improvement over the previous patch and sets readline_encoding to 'UTF-8' on Android when test_builtin is run with the environment variable LANG set to 'en_US.UTF-8' and in that case the test exercises all the code paths including those with the readline module. This is because locale.getdefaultlocale() returns ('en_US', 'UTF-8') on Android in that case and because both getdefaultlocale() and readline scan the environment to check for the locale. This new patch is only useful if tests on Android are expected to be run also with one of the locale environment variables set to UTF-8 (and PEP 538 is rejected).
I have left some comments on Rietveld.

bpo-31703 has been marked as a duplicate of this bug.

As well as other 5 bugs.

This old issue still is not fixed. Martin, Xavier, could you open a PR please?

As stated in msg285810, issue 28180 removed the need for an Android workaround. After checking this point on the Android API 24 emulator with the current master branch, I have opened PR 7133 which is Martin's input-readline.v3.patch with the Android workaround removed. The patch is commited with Martin as its author.

All checks have passed on PR 7133, even on macOs, and the PR is ready to be reviewed :-)

On 3.11 I don't see a problem on windows, but on mac test_input_tty from test_builtin hangs when test_readline runs first, while test_builtin passes on its own.

See also Issue44887.

On Fedora 35, I still reproduce the initial issue on the main branch of Python:
$ ./python -E -m test -v test_readline test_builtin
(...)
======================================================================
FAIL: test_input_tty_non_ascii (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2095, in test_input_tty_non_ascii
 self.check_input_tty("prompté", b"quux\xe9", "utf-8")
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2086, in check_input_tty
 self.assertEqual(input_result, expected)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
? +
======================================================================
FAIL: test_input_tty_non_ascii_unicode_errors (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2099, in test_input_tty_non_ascii_unicode_errors
 self.check_input_tty("prompté", b"quux\xe9", "ascii")
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2086, in check_input_tty
 self.assertEqual(input_result, expected)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
? +
(...)
Fedora 35 uses readline 8.1:
$ ldd $(./python -c 'import readline; print(readline.__file__)')
	...
	libreadline.so.8 => /lib64/libreadline.so.8 (0x00007fd00e553000)
	...
$ rpm -qf /lib64/libreadline.so.8
readline-8.1-3.fc35.x86_64
$ make pythoninfo|grep readline
readline._READLINE_LIBRARY_VERSION: 8.1
readline._READLINE_RUNTIME_VERSION: 0x801
readline._READLINE_VERSION: 0x801

Oh, the test_builtin.test_input_tty_non_ascii() fails just if test_readline is loaded previously:
$ ./python -E -m test -m test.test_builtin.PtyTests.test_input_tty_non_ascii -v test_readline test_builtin
== CPython 3.11.0a4+ (heads/main:7f4b69b9076, Jan 17 2022, 12:28:15) [GCC 11.2.1 20211203 (Red Hat 11.2.1-7)]
== Linux-5.15.12-200.fc35.x86_64-x86_64-with-glibc2.34 little-endian
== cwd: /home/vstinner/python/main/build/test_python_49429æ
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
0:00:00 load avg: 0.48 Run tests sequentially
0:00:00 load avg: 0.48 [1/2] test_readline
----------------------------------------------------------------------
Ran 0 tests in 0.000s
OK
0:00:00 load avg: 0.48 [2/2] test_builtin -- test_readline ran no tests
test_input_tty_non_ascii (test.test_builtin.PtyTests) ... FAIL
======================================================================
FAIL: test_input_tty_non_ascii (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2095, in test_input_tty_non_ascii
 self.check_input_tty("prompté", b"quux\xe9", "utf-8")
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2086, in check_input_tty
 self.assertEqual(input_result, expected)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
? +
----------------------------------------------------------------------
Ran 1 test in 0.013s
FAILED (failures=1)
test test_builtin failed
test_builtin failed (1 failure)
== Tests result: FAILURE ==
1 test failed:
 test_builtin
1 test run no tests:
 test_readline
Total duration: 559 ms
Tests result: FAILURE
In just, just importing readline is enough to make the test fails:
$ git diff Lib/test/test_builtin.py
diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
index 6dc4fa55502..20d3d33d9fb 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -1,3 +1,5 @@
+import readline
+
 # Python test set -- built-in functions
 
 import ast
$ ./python -E -m test -m test.test_builtin.PtyTests.test_input_tty_non_ascii -v test_builtin
(...)
======================================================================
FAIL: test_input_tty_non_ascii (test.test_builtin.PtyTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2097, in test_input_tty_non_ascii
 self.check_input_tty("prompté", b"quux\xe9", "utf-8")
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/vstinner/python/main/Lib/test/test_builtin.py", line 2088, in check_input_tty
 self.assertEqual(input_result, expected)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'quux' != 'quux\udce9'
- quux
+ quux\udce9
? +
(...)

Since nobody managed to fix this issue in 10 years and the test still fails if the readline module is loaded, I wrote GH-30631 to skip the test if the readline module is loaded.

rl-locale.diff changes the readline implementation of the PyOS_Readline() to set LC_CTYPE locale to "C": setlocale(LC_CTYPE, "C"), rather to the user preferred locale: setlocale(LC_CTYPE, ""). IMO it's a bad idea. Python made great progress in Unicode support, readline has a good Unicode support, and in most cases, it just works like a charm. This change looks a hack just to get these 2 specific tests to pass, but it breaks any other usage of readline.
rl-test.diff skips the test if the readline module can be imported, and it always import the readline module. It's different than that GH-30631 which only checks if the readline module is currently imported: my change doesn't import readline in test_builtin.
"input-readline*.patch" patches and GH-7133 spawn a fresh Python process to make sure that the readline is not imported or to import readline explicitly. They are better than my fix, but they are more complicated. It seems likle these changes also fix test_input_tty_non_ascii() but I don't understand how.

I marked bpo-41034 "test_builtin: PtyTests fail when run twice" as a duplicate of this issue. Moreover, I tested manually: my change GH-30631 fix the "./python -m test -R 3:3 test_builtin" command.

New changeset ad6e640f910787e73fd00f59117fbd22cdf88c78 by Victor Stinner in branch 'main':
bpo-13886: Skip PTY non-ASCII tests if readline is loaded (GH-30631)
https://github.com/python/cpython/commit/ad6e640f910787e73fd00f59117fbd22cdf88c78

With my change, the two following commands now pass successfully:
* ./python -m test test_readline test_builtin 
* ./python -m test -R 3:3 test_builtin

New changeset 1345b460f568afa8a6f9c0e2b23adba5015f208e by Miss Islington (bot) in branch '3.10':
bpo-13886: Skip PTY non-ASCII tests if readline is loaded (GH-30631)
https://github.com/python/cpython/commit/1345b460f568afa8a6f9c0e2b23adba5015f208e

New changeset 0fbb9afbddb93408e34bdb7625002374cb2ad68c by Miss Islington (bot) in branch '3.9':
bpo-13886: Skip PTY non-ASCII tests if readline is loaded (GH-30631) (GH-30635)
https://github.com/python/cpython/commit/0fbb9afbddb93408e34bdb7625002374cb2ad68c

Ok, the initial issue is now fixed: the test pass.
If someone wants to write test input() with non-ASCII input and readline, I suggest to open a new issue and add the test in the test_readline module instead.