support pdb.set_trace() with pytest-xdist

[pytestbot]

Originally reported by: Robert Buchholz (BitBucket: rbuchholz, GitHub: rbuchholz)

---

Given a test

#!python
...
    def test_should_not_create_duplicate_entries(self):
        from pdb import set_trace ; set_trace()
...

I get

#!shell
(virtualenv)2 rbu@localhost ~/code $ py.test
...
>>>>>>>>>>>>>>>>>>> PDB set_trace (IO-capturing turned off) >>>>>>>>>>>>>>>>>>>>
> some_test.py(48)test_should_not_create_duplicate_entries()
-> self.assert_increases_by(filler.fill, indexCache.query.count, 1)
(Pdb) c
................

(virtualenv)rbu@localhost ~/code $ py.test --looponfail  
...
>>>>>>>>>>>>>>>>>>> PDB set_trace (IO-capturing turned off) >>>>>>>>>>>>>>>>>>>>
> some_test.py(48)test_should_not_create_duplicate_entries()
-> self.assert_increases_by(filler.fill, indexCache.query.count, 1)
(Pdb) 
F...............
=================================== FAILURES ===================================
...
>           if self.quitting: raise BdbQuit
E           BdbQuit
################################ LOOPONFAILING #################################
...
############################# waiting for changes ##############################
### Watching:   /home/rbu/code
close failed in file object destructor:
sys.excepthook is missing
lost sys.stderr

This is py.test version 2.4.2, imported from .../pytest-2.4.2-py2.7.egg/pytest.pyc
setuptools registered plugins:
pytest-xdist-1.9 at .../pytest_xdist-1.9-py2.7.egg/xdist/plugin.pyc

---

• Bitbucket: https://bitbucket.org/pytest-dev/pytest/issue/390

[pytestbot]

Original comment by holger krekel (BitBucket: hpk42, GitHub: hpk42):

---

It's a known limitations that pytest-xdist and --pdb or pdb.set_trace() don't work. The former will error out immediately.

[pytestbot]

Original comment by Robert Buchholz (BitBucket: rbuchholz, GitHub: rbuchholz):

---

Hey Holger, thanks for the reply. I haven't found anything in the documentation or xdist's web/pypi page, so I did not know this whether was intentional or known.
Now we have it documented.

[pytestbot]

Original comment by BitBucket: dwt, GitHub: dwt:

---

Uhm, nice to know that this is expected. Could you perhaps elaborate on why that is so? Could this be changed? Do you want this to change?

Thanks!

[pytestbot]

Original comment by holger krekel (BitBucket: hpk42, GitHub: hpk42):

---

pytest-xdist manages a test slave process which collects and runs tests (at the control of the master process). However, it doesn't currently implement "stdout/stderr/stdin" forwarding. So it is this forwarding that is missing for making pdb() work. It's probably not very hard to implement this forwarding but requires understanding of execnet which is the underlying library for managing master-slave communication.

[rbu]

@rbuchholz is actually @rbu on GitHub

[RonnyPfannschmidt]

Closing as stalled, also this one is rather hard to do propperly

[astockwell]

If you need to use PDB momentarily to troubleshoot, but don't want to rip out xdist, you can invoke it with the flags pytest -n0 --dist no which disables xdist entirely for that run (this also fixes the -s (stdout redirect) flag). Source: pytest-dev/pytest-xdist#231

[ralokt]

Also, if you need to use a debugger, and you actually need test distribution (for example, because you are debugging a xdist-heisenbug that only happens when tests are distributed), you can use a remote debugger! python-remote-debug just solved that issue for me.

[sanmai-NL]

@astockwell That doesn‘t work for me:

$ uv run --env-file=.env -- pytest --pdb -n0 --dist=no --strict-config --strict-markers -vv
======================================================================================================================================================== test session starts =========================================================================================================================================================
platform darwin -- Python 3.13.2, pytest-8.3.4, pluggy-1.5.0 -- /Users/myuser/app/.venv/bin/python3
cachedir: .pytest_cache
rootdir: /Users/myuser/app
configfile: pyproject.toml
testpaths: tests
plugins: anyio-4.8.0, xdist-3.6.1

The xdist plugin is still loaded, and the test hangs upon calling breakpoint().