Specifying which Python interpreter stubgen should use

[JukkaL]

Add a -p <path-to-python> option to stubgen that will replace the --py2 option. Mypy would use this interpreter for these things:

• Determining Python version (2 or 3).
• Determining Python module search path.
• Runtime introspection (esp. for generating stubs for C modules).

As pointed out by @gnprice, the current behavior of using the current Python interpreter doesn't make sense when using a virtualenv, as the virtualenv would have mypy dependencies instead of the (third party) modules targeted by stubgen. This would also allow using an arbitrary Python 2 virtualenv.

We may also need a way of specifying PYTHONPATH for the target interpreter separately from the PYTHONPATH used to run mypy.

[gnprice]

This came to mind again today, in the context not of stubgen but of mypy itself. Someone on IRC had trouble getting mypy to find the code they were type-checking, because they were depending on a virtualenv and so --use-python-path didn't help them. If we had a -p option, they could use that in the same way for mypy as we were discussing for stubgen.

Would it make sense for the same -p option to apply to both? Perhaps we should retitle this bug to include both.

[JukkaL]

Good point -- retitled.

[ethanhs]

#4693 added this for mypy.

[hauntsaninja]

stubgen gained support for this at some point in the last four years