I've never used uv before, but this was a good opportunity to give it a try:

• uv python install
• uv venv
• uv pip install -e .

All that took about a second, which is indeed pretty damn crazy 🤯

And indeed I can reproduce with that. A breakpoint() in a normal Python file (outside of pytest) seems to work fine. And disabling pytest's output capturing with -s seems to fix things too!

Can this be related to the readline quirk documented @ https://gregoryszorc.com/docs/python-build-standalone/main/quirks.html#backspace-key-doesn-t-work-in-python-repl ?

Hmm, I don't think so. I don't see such an error message, and for what it's worth, setting TERMINFO_DIRS=/etc/terminfo:/lib/terminfo:/usr/share/terminfo doesn't change anything either.

Oh, this one seems like a likely culprit, however:

Use of libedit on Linux

Python 3.10+ Linux distributions link against libedit (as opposed to readline) by default, as libedit is supported on 3.10+ outside of macOS.

Most Python builds on Linux will link against readline because readline is the dominant library on Linux.

Some functionality may behave subtly differently as a result of our choice to link libedit by default. (We choose libedit by default to avoid GPL licensing requirements of readline.)

Perhaps something similar to #1281 could be going on with libedit (or their usage of it) too?

Yep, it's that indeed. Built a vanilla CPython 3.13.0 with ./configure --with-readline=editline and it behaves in the exact same way. It would also explain why @jaraco saw this on macOS over in #12497 (comment) because from what I understand, CPython uses it on macOS by default.

Tried CPython v3.12.7 built the same way, that works. Will now bisect between the two...

Well, this was a bit annoying as two build fixes for readline were in between:

cpython$ git log --oneline v3.12.7..v3.13.0 -- Modules/readline.c
417dd3aca7b gh-116322: Rename PyModule_ExperimentalSetGIL to PyUnstable_Module_SetGIL (GH-118645)
c2627d6eea9 gh-116322: Add Py_mod_gil module slot (#116882)
!! 8515fd79fef gh-117845: Detect libedit hook function signature in configure (#117870)
e7e1116a781 gh-105323: Remove `WITH_APPLE_EDITLINE` to use the same declaration for all editline (gh-112513)
c2982380f82 gh-112510: Add `readline.backend` for the backend readline uses (GH-112511)
2df26d83486 gh-112105: Make completer delims work on libedit (gh-112106)
154f099e611 gh-112292 : Catch import error conditions with readline hooks (gh-112313)
!! f4cb0d27cc0 gh-109191: Fix build with newer editline (gh-110239)
501939c9c14 gh-105323: Update readline module to detect apple editline variant (gh-108665)
e7de0c5901b gh-108765: Python.h no longer includes <sys/time.h> (#108775)
b32d4cad15f gh-108444: Replace _PyLong_AsInt() with PyLong_AsInt() (#108459)
d228825e088 gh-106320: Remove _PyOS_ReadlineTState API (#107034)
c9ce983ae1a gh-106320: Remove private pylifecycle.h functions (#106400)
a9305b5e80e gh-105240: add missing function prototypes (#105241)
9ab587b7146 gh-89886: Rely on HAVE_SYS_TIME_H (#105058)

but with a bit of git cherry-pick to get them to whatever commit I was testing, I was able to figure out the culprit:

commit 01e7405da400e8997f8964d06cc414045e144681 (HEAD)
Author: Tian Gao <[email protected]>
Date:   Mon Mar 25 08:18:09 2024 -0700

    gh-112948: Make pdb completion similar to repl completion (#112950)

 Lib/pdb.py                                                              | 43 ++++++++++++++++++++++++++++---------------
 Lib/test/test_pdb.py                                                    | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 Misc/NEWS.d/next/Library/2023-12-11-00-51-51.gh-issue-112948.k-OKp5.rst |  1 +
 3 files changed, 80 insertions(+), 15 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2023-12-11-00-51-51.gh-issue-112948.k-OKp5.rst

that is:

• Make pdb completion similar to repl completion python/cpython#112948
• gh-112948: Make pdb completion similar to repl completion python/cpython#112950

which notably seems to use rlcompleter now in pdb.

Not sure where to go from here. If we want to open a CPython bug about this, we probably need a more minimal reproducer? But I have no clue whether the culprit is CPython or pytest here.

@The-Compiler wow that's some impressive investigative skills right there! I certainly wouldn't have been able to figure this all out so fast...

All that took about a second, which is indeed pretty damn crazy 🤯

Yeah it's pretty sweet ^^

... disabling pytest's output capturing with -s seems to fix things too!

@The-Compiler my hero. My sanity owes you a debt of gratitude!

Chiming in to confirm that I've been experiencing this bug as well, specifically in a uv-generated venv (but haven't checked using any other package managers):

• kernel 6.8
• python 3.13
• uv 0.4.25

@The-Compiler similar bug happen in gdb: (because of python/cpython#112948 )

$ gdb
(gdb) st<tab>
start     starti    step      stepi     stepping  stop      strace
(gdb) pi
>>> [o for o in sys.modules.keys() if 'readline' in o]
[]
>>> import pdb
>>> [o for o in sys.modules.keys() if 'readline' in o]
['readline']
>>> quit

(gdb) st<tab><tab>

After import pdb in py3.13, autocomplete is broken

Someone (not me) will need to report this to gdb, and ideally cook up a reproducer and report it to CPython and/or libedit. I won't be spending any more time on this, as it doesn't affect me and we have a workaround for pytest.