Unittest test discovery fails for Python 3.15

[pewscorner]

Type: Bug

Behaviour

The testing panel fails to list more than one unittest test when using the latest Python 3.15.0 prerelease (a6).

Steps to reproduce:

1. Open a folder containing unittest tests.
2. Set the Python interpreter to Python 3.15.0a6.
3. Check the list of tests in the testing panel. This list will be incomplete (only the first test will be visible), and the Python output log will show an error (see below).

Note that test discovery works just fine outside VS Code, e.g. using the following command:

py -3.15 -m unittest discover -v -s . -p "*unittest.py"

Example of failed test discovery with Python 3.15.0a6

Example of successful test discovery with Python 3.14.3

Diagnostic data

Output for Python in the Output panel (View→Output, change the drop-down the upper-right of the Output panel to Python)

2026-02-18 01:27:44.521 [info] Environment variables set for unittest discovery: TEST_RUN_PIPE=\\.\pipe\python-test-discovery-5f89198e3aad4f29231c
2026-02-18 01:27:44.521 [info] Using execution factory for unittest discovery in workspace c:\Users\user\Documents\PythonProjects\PythonQuickRefTest
2026-02-18 01:27:44.530 [info] > "C:\Program Files\Python315\python.exe" ~\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\python_files\unittestadapter\discovery.py --udiscovery -v -s ./unittests -p *unittest.py
2026-02-18 01:27:44.530 [info] cwd: .
2026-02-18 01:27:44.530 [info] Started unittest discovery subprocess (execution factory) for workspace c:\Users\user\Documents\PythonProjects\PythonQuickRefTest
2026-02-18 01:27:44.696 [error] Test Discovery named pipe \\.\pipe\python-test-discovery-5f89198e3aad4f29231c error: [TypeError: Cannot read properties of undefined (reading 'forEach')
	at e (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:993915)
	at c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:994918
	at Array.forEach (<anonymous>)
	at e (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:993915)
	at c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:994918
	at Array.forEach (<anonymous>)
	at e (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:993915)
	at c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:994918
	at Array.forEach (<anonymous>)
	at e (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:993915)
	at t.TestDiscoveryHandler.processDiscovery (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:970224)
	at u.resolveDiscovery (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:967007)
	at c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:960124
	at _._callback (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:992938)
	at b.callback (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:227303)
	at Object.thunk (c:\Users\user\.vscode\extensions\ms-python.python-2026.2.0-win32-x64\out\client\extension.js:2:2069243)]
2026-02-18 01:27:44.708 [info] Unittest discovery completed for workspace c:\Users\user\Documents\PythonProjects\PythonQuickRefTest

Extension version: 2026.2.0
VS Code version: Code 1.109.4 (c3a26841a84f20dfe0850d0a5a9bd01da4f003ea, 2026-02-16T15:35:57.932Z)
OS version: Windows_NT x64 10.0.26200
Modes:

System Info

Item	Value
CPUs	Intel(R) Core(TM) i9-14900K (32 x 3187)
GPU Status	2d_canvas: enabled GPU0: VENDOR= 0x10de, DEVICE=0x2803 [NVIDIA GeForce RTX 4060 Ti], DRIVER_VENDOR=NVIDIA, DRIVER_VERSION=32.0.15.8104 ACTIVE GPU1: VENDOR= 0x8086, DEVICE=0xa780 [Intel(R) UHD Graphics 770], DRIVER_VERSION=32.0.101.7077 GPU2: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.7309 Machine model name: Machine model version: 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 trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	61.66GB (45.27GB free)
Process Argv
Screen Reader	no
VM	0%

[eleanorjboyd]

will investigate thanks