Use default python_executable when pythonPath is not a filepath

[travbid]

I have a python file:

import grpc

After installing the dependency via pip3 install grpcio, the mypy-vscode extension reports this error example.py:1:1: error: Cannot find implementation or library stub for module named 'grpc'

This happens because:

• By default, VSCode sets "python.pythonPath" to "python"

• mypy_server.py fetches this value in confiuration_changed()

• got_python_executable() decides that this is not a file that can be executed and sets python_executable to None.

• start_server_and_analyze() then sets options.python_executable from its default of sys.executable to that value.

• Because options.python_executable is None, modulefinder.py in mypy simply returns [] for site-packages: https://github.com/python/mypy/blob/63f2fed5d0e10d4d875442e9bba2283c81dfdf4b/mypy/modulefinder.py#L465

• Thus, mypy cannot find my dependency and returns: example.py:1:1: error: Cannot find implementation or library stub for module named 'grpc'

This commit only overrides the default options.python_executable if ours is not None. I don't know if this is the best way to solve the problem, but I think leaving options.python_executable as its default is a reasonable thing to do.

[matangover]

Thanks for the PR! Good catch. I will take a closer look soon. I'm hesitating to accept because IIUC this means that in the default case, this may create confusion: the user will install a module to system Python, but mypyls won't find it because it would use the Python on which it is installed (a virtualenv, if you follow my recommended installation instructions). The only 'correct' solution I guess is to expand 'python' using the system path, mimicking the behavior of vscode-python extension. In fact, vscode-python is now in the process of deprecating the pythonPath setting...

[matangover]

@travbid Thanks very much for debugging this issue and for the PR. I agree your solution is good for now. Going forward, the best would be to use the new API from the Python extension to get the executable path:

https://github.com/microsoft/pyright/issues/697 https://github.com/formulahendry/vscode-code-runner/issues/604 https://github.com/microsoft/vscode-python/issues/11294

Anyway users can always just configure their path in mypy.ini, this whole interpreter selection in VSCode is quite a mess...

[karrtikr]

@matangover This code snippet shows how to use the new API, could be helpful.