PYTHONPATH is corrupted by bundled dependencies dir in mypy process when using mypy-type-checker.path

[edwardpeek-crown-public]

When mypy-type-checker.path is configured to use a user-controlled version of mypy instead of the bundled one, it is invoked with the PYTHONPATH env var set to include the extension's bundled python dependencies which causes mypy to return unexpected results as it loads dependencies from the bundled libs in preference to venv or system equivalents.

Configuring this setting with a shell stub to print environment such as

#!/bin/sh
env | sort 1>&2
exec ./venv/bin/mypy "$@"

reveals

PYTHONPATH=<redacted>/.vscode/extensions/ms-python.mypy-type-checker-2023.6.0/bundled/libs:

in the extension's output panel.

The extension setting importStrategy=fromEnvironment is affected by a similar bug: microsoft/vscode-black-formatter#369

The other VS Code extensions for black, isort, pylint do not set PYTHONPATH and so are correctly isolated from the LSP server python environment.

[karthiknadig]

We need to add a check to verify that the mypy provided via path succeeds. If not, then we can leave an error log. Attempting fallback with PYTHONPATH can lead to same problem of loading packages from the wrong place.

[edwardpeek-crown-public]

FWIW this issue can be worked around by VS Code settings similar to:

  "mypy-type-checker.path": [
    // Workaround for https://github.com/microsoft/vscode-mypy/issues/217
    "/usr/bin/env",
    "--unset=PYTHONPATH",
    "${workspaceFolder}/.venv/bin/mypy",
  ],