VSCode pytest data from WSL generate an error when processed

[KinkosPouet]

Type: Bug

Behaviour

Pytesting through UI doesn't work with WSL extension. An error occures when processing data sent from WSL to VScode :

Steps to reproduce:

1. Create environnement with hatch (
   hatch env run -e dev test)
2. Select interpreter in VSCode to point on the venv dir ( hatc
   h env find dev)
3. Go to testing in VSCode, and play the test.
4. Test pass without error, but the data is not sent back

Output for Python in the Output panel

Running pytest with args: ['-p', 'vscode_pytest', '--rootdir=/home/user/git/', '/home/user/git/app/tests/test_main.py::test_load_config', '/home/user/git/app/tests/test_main.py::test_get_db_connection_postgresql', '/home/user/git/app/tests/test_main.py::test_get_db_connection_mysql', '/home/user/git/app/tests/test_main.py::test_is_database_empty_postgresql', '/home/user/git/app/tests/test_main.py::test_is_database_empty_mysql', '/home/user/git/app/tests/test_main.py::test_execute_sql_file', '/home/user/git/app/tests/test_main.py::test_process_database_when_empty', '/home/user/git/app/tests/test_main.py::test_process_database_exception', '/home/user/git/app/tests/test_main.py::test_main_process_multiple_databases']
============================= test session starts ==============================
platform linux -- Python 3.11.2, pytest-7.4.3, pluggy-1.5.0
rootdir: /home/user/git/
plugins: mock-3.14.0
collected 9 items

app/tests/test_main.py Plugin error, exception thrown while attempting to send data[vscode-pytest]: unsupported operand type(s) for +=: 'int' and 'NoneType' 
[vscode-pytest] data: 
{"jsonrpc": "2.0", "params": {"cwd": "/home/user/git/", "status": "success", "result": {"/home/user/git/app/tests/test_main.py::test_load_config": {"test": "/home/user/git/app/tests/test_main.py::test_load_config", "outcome": "success", "message": null, "traceback": null, "subtest": null}}, "not_found": null, "error": null}}

(X9)

.           [100%]

============================== 9 passed in 0.05s ===============================
Finished running tests!

Extension version: 2025.0.0
VS Code version: Code 1.97.0 (33fc5a94a3f99ebe7087e8fe79fbe1d37a251016, 2025-02-04T22:41:26.688Z)
OS version: Windows_NT x64 10.0.26100
Modes:
Remote OS version: Linux x64 5.15.167.4-microsoft-standard-WSL2
Remote OS version: Linux x64 5.15.167.4-microsoft-standard-WSL2
Remote OS version: Linux x64 5.15.167.4-microsoft-standard-WSL2

• Python version (& distribution if applicable, e.g. Anaconda): 3.11.2
• Type of virtual environment used (e.g. conda, venv, virtualenv, etc.): Venv
• Value of the python.languageServer setting: Default

User Settings

languageServer: "Pylance"

testing
• pytestArgs: "<placeholder>"
• pytestEnabled: true

Installed Extensions

Extension Name	Extension Id	Version
debugpy	ms-	2025.0.0
gitlens	eam	16.3.0
js-debug	ms-	1.97.0
python	ms-	2025.0.0
vscode-js-profile-table	ms-	1.0.10
vscode-pylance	ms-	2025.2.1

System Info

Item	Value
CPUs	Intel(R) Core(TM) Ultra 9 185H (22 x 3072)
GPU Status	2d_canvas: enabled canvas_oop_rasterization: enabled_on direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off video_decode: enabled video_encode: enabled vulkan: disabled_off webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	31.46GB (12.42GB free)
Process Argv	--crash-reporter-id bb4a69d2-a4d3-44a5-b62b-19c8f9b82470
Screen Reader	no
VM	0%

Item	Value
Remote	WSL: Debian
OS	Linux x64 5.15.167.4-microsoft-standard-WSL2
CPUs	Intel(R) Core(TM) Ultra 9 185H (22 x 0)
Memory (System)	15.35GB (13.51GB free)
VM	0%

Item	Value
Remote	WSL: Debian
OS	Linux x64 5.15.167.4-microsoft-standard-WSL2
CPUs	Intel(R) Core(TM) Ultra 9 185H (22 x 0)
Memory (System)	15.35GB (13.51GB free)
VM	0%

Item	Value
Remote	WSL: Debian
OS	Linux x64 5.15.167.4-microsoft-standard-WSL2
CPUs	Intel(R) Core(TM) Ultra 9 185H (22 x 0)
Memory (System)	15.35GB (13.51GB free)
VM	0%

A/B Experiments

vsliv368cf:30146710
vspor879:30202332
vspor708:30202333
vspor363:30204092
vscod805cf:30301675
binariesv615:30325510
py29gd2263:31024239
c4g48928:30535728
azure-dev_surveyone:30548225
962ge761:30959799
2e7ec940:31000449
pythontbext0:30879054
cppperfnew:31000557
dwnewjupytercf:31046870
nativerepl1:31139838
pythonrstrctxt:31112756
nativeloc1:31192215
iacca1:31171482
5fd0e150:31155592
dwcopilot:31170013
stablechunks:31184530
6074i472:31201624
9064b325:31222308
copilot_t_ci:31222730
jda6j935:31233686

[eleanorjboyd]

hm looks like it is this line causing the issue bytes_written += __writer.write(segment) from here. Not sure why looks like there is just a problem writing to the named pipe that is open for communication. @karthiknadig any ideas here specifically since this is coming from WSL? I can also add in some more logging, potentially the name-pipe vs use of fifo is being incorrectly selected on WSL?

[MartinSalinas98]

I'm having a very similar issue since fairly recently. I haven't isolated the specific plugin version, but, in my specific case, seems to happen when running (individually and from UI) unitary tests that make use of mock_open, which makes sense if the involved lines contain an open() call.

Some time ago I opened issue #24399, but the behaviour described in that issue seems to have been replaced by the one described here.

[KinkosPouet]

FYI i did this on a brand new environnement on a new computer.
My code didn't change between my migration from old computer to new one.
I Installed WSL + got my code from my git repo without any changes.

[karthiknadig]

@eleanorjboyd We need to repro this locally to see what is the error. It is likely a path issue, if the paths are /mnt/c/ style then the vs /tmp then there could be a difference in how we communicate with those. We need to see the logs on how the paths look like. we need to make sure that in WSL scenarios the path does not use Windows mounted folders for pipes.

[KinkosPouet]

@karthiknadig To add some information, in my case, every python files used were in /home/user/, nothing was put in /mnt/c.

[karthiknadig]

The issue is not with the python files themselves, we use pipes to communicate between VS Code and pytest. When we create the pipes, in WSL they should be created in /tmp and not in any location that is /mnt/. My concern is that it might be creating it in the wrong location, leading to a bad pipe stream.