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

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

languageServer: "Pylance"

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

experiments
• optInto: ["pythonTestAdapter"]

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