Allow -n 0 to disable xdist even if --dist is specified

[timj]

Following the advice in #18 I'm experimenting with loadscope as a means to to pin some tests to a single subprocess. This works fine but to ensure that it works properly for developers who are going to type pytest from the command line when debugging, I need to put the --dist=loadscope into a setup.cfg file. The problem I now find is that any use of --dist mandates that -n or -tx is used:

$ pytest -v  --dist=load --fulltrace  tests/test_register.py 
============================= test session starts ==============================
platform darwin -- Python 3.6.0, pytest-3.2.0, py-1.4.34, pluggy-0.4.0 -- /Users/timj/work/lsstsw3/miniconda/bin/python
cachedir: .cache
rootdir: /Volumes/G-RAID with Thunderbolt/transient/lsstsw3/build/pipe_tasks, inifile: setup.cfg
plugins: session2file-0.1.9, forked-0.3.dev0+g1dd93f6.d20170815, xdist-1.19.2.dev0+g459d52e.d20170815, flake8-0.8.1
ERROR: MISSING test execution (tx) nodes: please specify --tx

I would be fine with this if I could add -n 0 to disable xdist completely regardless of the presence of --dist. This does not work, and gives the same error message as if no -n was used at all. If this worked then the command-line user of pytest will get the behavior they expect: no subprocess if they run pytest, correctly scoped tests if they add -n 4 to override the config file.

Is there any way that I can use a setup.cfg to use --dist=loadscope but only to activate this if a user specifies -n?

[souliane]

I am concerned by this issue / change request, since I can't use a debugger when pytest-xdist runs with the --dist parameter set in my setup.cfg. I would expect that -n 0 completely disables xdist.

[kousu]

I have this in my setup.cfg:

[tool:pytest]
addopts = --verbose --numprocesses auto --dist=loadscope
python_files = unit_testing/test_*.py unit_testing/cli/test_*.py

Running

 pytest -n0 -s -v .
====================================================================================== test session starts =======================================================================================
platform linux -- Python 3.6.12, pytest-6.2.1, py-1.10.0, pluggy-0.13.1 -- /home/kousu/src/neuropoly/spinalcordtoolbox/python/envs/venv_sct/bin/python
cachedir: .pytest_cache
rootdir: /home/kousu/src/neuropoly/spinalcordtoolbox, configfile: setup.cfg
plugins: forked-1.3.0, cov-2.10.1, console-scripts-1.1.0, xdist-2.2.0, cases-3.1.1
ERROR: MISSING test execution (tx) nodes: please specify --tx

If I change it to

[tool:pytest]
addopts = --verbose
python_files = unit_testing/test_*.py unit_testing/cli/test_*.py

Then it works:

$ pytest -n0 -s -v .
====================================================================================== test session starts =======================================================================================
platform linux -- Python 3.6.12, pytest-6.2.1, py-1.10.0, pluggy-0.13.1 -- /home/kousu/src/neuropoly/spinalcordtoolbox/python/envs/venv_sct/bin/python
cachedir: .pytest_cache
rootdir: /home/kousu/src/neuropoly/spinalcordtoolbox, configfile: setup.cfg
plugins: forked-1.3.0, cov-2.10.1, console-scripts-1.1.0, xdist-2.2.0, cases-3.1.1
collecting 14 items                                                                                                                                                                              
--
Spinal Cord Toolbox (git-ng/2957-fixtures-7bbce7ab7c113c0d9725fdf7ab8406ea432a9ae7*)

Smoothing factor: smooth=0.3703703703703704
Smoothing factor: smooth=0.3703703703703704
Smoothing factor: smooth=0.3703703703703704
Smoothing factor: smooth=0.3703703703703704
Smoothing factor: smooth=0.3703703703703704
Smoothing factor: smooth=0.3703703703703704
Smoothing factor: smooth=0.3703703703703704
Smoothing factor: smooth=0.011999999999999999
Smoothing factor: smooth=0.011999999999999999
Smoothing factor: smooth=0.011999999999999999
Smoothing factor: smooth=0.011999999999999999
Smoothing factor: smooth=0.011999999999999999
collecting 57 items                                                                                                                                                                              (private) matplotlib data path: /home/kousu/src/neuropoly/spinalcordtoolbox/python/envs/venv_sct/lib/python3.6/site-packages/matplotlib/mpl-data
matplotlib data path: /home/kousu/src/neuropoly/spinalcordtoolbox/python/envs/venv_sct/lib/python3.6/site-packages/matplotlib/mpl-data
CONFIGDIR=/home/kousu/.config/matplotlib
matplotlib version 3.3.3
interactive is False
platform is linux
[...]

I also expected -n 0 to completely disable xdist. But that's not totally true, is it, since eg the worker_id fixture still works.

[MetRonnie]

If you do pytest -n0 --dist no it seems to prevent the

ERROR: MISSING test execution (tx) nodes: please specify --tx

error

[timj]

It looks like this problem was fixed long ago. Running pytest --dist=loadscope handles no -n just fine.

[nicoddemus]

Thanks!