pytest.HIDDEN_PARAM Causes Discovery Failure

[tobias-westerheide]

Type: Bug

Using pytest.HIDDEN_PARAM in parametrized tests causes pytest discovery to fail with a ValueError in the VS Code Python extension's test discovery mechanism.

Steps to reproduce:

1. Create a test file with a parametrized test using pytest.HIDDEN_PARAM:

import pytest

@pytest.mark.parametrize("setup", [pytest.param("setup", id=pytest.HIDDEN_PARAM)])
def test_main(setup) -> None:
    assert True

2. Open the file in VS Code with the Python extension enabled
3. Observe that pytest discovery fails

Diagnostic data

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

pytest test discovery error for workspace:  /tmp/test_pytest 
 Error Occurred, traceback: Traceback (most recent call last):
  File "/home/tobi/.vscode-server/extensions/ms-python.python-2026.0.0/python_files/vscode_pytest/__init__.py", line 428, in pytest_sessionfinish
    session_node: TestNode | None = build_test_tree(session)
                                    ~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/tobi/.vscode-server/extensions/ms-python.python-2026.0.0/python_files/vscode_pytest/__init__.py", line 691, in build_test_tree
    test_node = process_parameterized_test(
        test_case, test_node, function_nodes_dict, file_nodes_dict
    )
  File "/home/tobi/.vscode-server/extensions/ms-python.python-2026.0.0/python_files/vscode_pytest/__init__.py", line 617, in process_parameterized_test
    parent_part, parameterized_section = test_node["name"].split("[", 1)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: not enough values to unpack (expected 2, got 1)

Extension version: 2026.0.0
VS Code version: Code 1.109.3 (b6a47e94e326b5c209d118cf0f994d6065585705, 2026-02-12T17:54:48.470Z)
OS version: Windows_NT x64 10.0.26200
Modes:
Remote OS version: Linux x64 5.15.153.1-microsoft-standard-WSL2
Remote OS version: Linux x64 5.15.153.1-microsoft-standard-WSL2
Remote OS version: Linux x64 5.15.153.1-microsoft-standard-WSL2

• Python version (& distribution if applicable, e.g. Anaconda): 3.11.11
• 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
• unittestArgs: "<placeholder>"

experiments
• optInto: ["pythonTestAdapter"]

Installed Extensions

Extension Name	Extension Id	Version
ansible	red	26.1.3
auto-commit-msg	Mic	0.27.2
autodocstring	njp	0.6.1
change-case	wma	1.0.0
code-spell-checker	str	4.5.6
color-highlight	nau	2.8.0
copilot-chat	Git	0.37.6
debugpy	ms-	2025.18.0
direnv	mkh	0.17.0
djlint	mon	2025.11.0
EditorConfig	Edi	0.18.1
even-better-toml	tam	0.21.2
gitignore	cod	0.10.0
gitlab-workflow	Git	6.69.3
gitlens	eam	17.10.0
gitlink	qez	1.2.6
go	gol	0.52.2
isort	ms-	2025.0.0
jinjahtml	sam	0.20.0
js-debug	ms-	1.105.0
jupyter	ms-	2025.9.1
jupyter-renderers	ms-	1.3.0
live-server	ms-	0.4.17
markdown-all-in-one	yzh	3.6.3
markdown-pdf	yza	1.5.0
markdowntable	Tak	0.13.0
marp-vscode	mar	3.3.0
mypy-type-checker	ms-	2025.2.0
oxc-vscode	oxc	1.45.0
pdminterpreter	Kal	0.0.5
plantuml	jeb	2.18.1
prettier-vscode	esb	12.3.0
python	ms-	2026.0.0
ruff	cha	2026.36.0
shellcheck	tim	0.38.6
tfsec	tfs	1.11.0
unique-window-colors	stu	1.0.51
vscode-containers	ms-	2.4.1
vscode-conventional-commits	viv	1.27.0
vscode-coverage-gutters	rya	2.14.0
vscode-django	bat	1.15.0
vscode-js-profile-table	ms-	1.0.10
vscode-jupyter-cell-tags	ms-	0.1.9
vscode-jupyter-slideshow	ms-	0.1.6
vscode-opentofu	ope	0.6.0
vscode-pylance	ms-	2025.10.4
vscode-python-envs	ms-	1.16.0
vscode-stylelint	sty	2.0.2
vscode-wakatime	Wak	25.5.1
vscode-yaml	red	1.19.1
vsliveshare	ms-	1.0.5959

System Info

Item	Value
CPUs	Intel(R) Core(TM) Ultra 7 165U (14 x 2688)
GPU Status	2d_canvas: enabled GPU0: VENDOR= 0x8086, DEVICE=0x7d45 [Intel(R) Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.7026 ACTIVE GPU1: 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)	63.46GB (35.34GB free)
Process Argv	--folder-uri=vscode-remote://wsl+Ubuntu-24.04/home/tobi/projects/gitlab.energiekoppler.com/ek/software/ee-redispatch --remote=wsl+Ubuntu-24.04
Screen Reader	yes
VM	0%

Item	Value
Remote	WSL: Ubuntu-24.04
OS	Linux x64 5.15.153.1-microsoft-standard-WSL2
CPUs	Intel(R) Core(TM) Ultra 7 165U (14 x 0)
Memory (System)	31.08GB (18.75GB free)
VM	0%

Item	Value
Remote	WSL: Ubuntu-24.04
OS	Linux x64 5.15.153.1-microsoft-standard-WSL2
CPUs	Intel(R) Core(TM) Ultra 7 165U (14 x 0)
Memory (System)	31.08GB (18.75GB free)
VM	0%

Item	Value
Remote	WSL: Ubuntu-24.04
OS	Linux x64 5.15.153.1-microsoft-standard-WSL2
CPUs	Intel(R) Core(TM) Ultra 7 165U (14 x 0)
Memory (System)	31.08GB (18.83GB free)
VM	0%

Extended Info

Root Cause

The issue is in the process_parameterized_test function at line 617 of vscode_pytest/__init__.py:

parent_part, parameterized_section = test_node["name"].split("[", 1)

This code assumes that all parametrized tests have a [ character in their name (e.g., test_main[setup]). However, when using pytest.HIDDEN_PARAM, pytest intentionally omits the parameter brackets from the test ID, resulting in just test_main.

The .split("[", 1) call then returns a list with only one element (the test name without brackets), but the code tries to unpack it into two variables, causing the ValueError.

Suggested Fix

The process_parameterized_test function should check if the test name contains parameters before attempting to split:

if "[" in test_node["name"]:
    parent_part, parameterized_section = test_node["name"].split("[", 1)
    # Process parameterized test
else:
    # Handle test without visible parameters (e.g., HIDDEN_PARAM)
    parent_part = test_node["name"]
    # Treat as non-parameterized or handle appropriately

Additional Context

• pytest.HIDDEN_PARAM is a documented pytest feature for hiding parameter values from test IDs
• This is particularly useful for fixture setup that shouldn't be visible in test names
• Command-line pytest handles this correctly; only VS Code discovery fails
• Extension logs show collected 1 item before the error, suggesting pytest itself collected the test successfully

Workaround

Remove the use of pytest.HIDDEN_PARAM and use a regular string ID instead:

@pytest.mark.parametrize("setup", [pytest.param("setup", id="setup")])
def test_main(setup) -> None:
    assert True

However, this defeats the purpose of HIDDEN_PARAM and creates visible parameter IDs.

References

• pytest documentation on HIDDEN_PARAM: https://docs.pytest.org/en/stable/reference/reference.html#pytest.HIDDEN_PARAM