#14014 broke mypy integration in combination with custom mypy plugins and editable installs

[elprans]

Environment data

• VS Code version: 1.52.1
• Extension version (available under the Extensions sidebar): v2020.12.424452561
• OS and version: Linux
• Python version (& distribution if applicable, e.g. Anaconda): Python 3.8
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): venv
• Relevant/affected Python packages and their versions: all of them

We use a custom mypy plugin in a project that is also commonly installed in "editable mode" (pip install -e). Editable mode installs do not link files to site-packages and instead populate the easy-install.pth file, which is used to populate sys.path.

Since the change in #14014, the extension is removing the value matching that of the workspace directory, which would match the value in easy_install.pth, effectively removing the entire project from sys.path and making the plugins invisible to mypy, which will make it crash on launch.

Note that this behavior isn't specific to mypy. Any similar customization of a lint tool (e.g. flake8) is similarly broken.

[elprans]

If I may, a better fix for the "don't add the script directory to sys.path" would be to run pythonFiles/pyvsc-run-isolated.py using some random empty working directory, and then restoring the correct cwd with os.chdir().

[brettcannon]

This is a duplicate of #14570 (and thanks for the suggestion of the fix!).