Make `--explicit-package-bases` revertible

[gustavgransbo]

Feature

--explicit-package-bases should have an inverse: --no-explicit-package-bases.

Pitch

I have a mypy.ini that declares explicit_package_bases = True. It would be convenient to be able to override this setting from the command line by passing --no-explicit-package-bases.

My use case is that I have a mono repo with a shared mypy.ini, where I mostly want to use --explicit-package-bases.
However, in some sub directories I have the following structure:

sub-dir
│
└───src
│   └───pkg
│   │   └─── # Package content
│   └───tests
│
└───noxfile.py

The noxfile.py is used to trigger a mypy src/ tests/ --config {PATH_TO_CONFIG} session with sub-dir as working directory.
I would like to use the same mypy.ini I use in the rest of the repo to not have to duplicate settings, but this sets --explicit-package-bases. Unfortunately this leads to mypy thinking that src is the namespace of pkg, since it is located at the working directory. The most convenient solution for me would be to pass --no-explict-package-bases when invoking mypy in sub-dir.

[hauntsaninja]

I might be misunderstanding, but I'm not sure that would help if pkg is actually a namespace package, since mypy wouldn't correctly map files to module names. Try putting src in your mypy path instead? (on the lines of the example in point 3 of https://mypy.readthedocs.io/en/latest/running_mypy.html#mapping-file-paths-to-modules)

[gustavgransbo]

Sorry, maybe the example was not clear enough.

pkg is not a namespace package, but when mypy is invoked with the working directory sub-dir, it thinks that pkg is a namespace package. I would like to be able to explicitly turn of explicit-namespace-packages when I run mypy in sub-dir.

[gustavgransbo]

Since it was such a small addition I went ahead and made an attempt at implementing this.

Maybe my use case seems slim, but since you already support inverting many other flags, I hope you wont mind supporting this specific flag.

One thing that had me a little bit perplexed was why the flag is part of the Running code section. It seems like it would be more at home in Import discovery, next to --namespace-packages.

[gustavgransbo]

Try putting src in your mypy path instead?

Actually, this should solve my use case regardless of pkg not being a namespace package, thank you for the suggestion!

I think having the option to override the config could still be useful though. Adding src to mypy_path would potentially affect other parts of the repository, whereas explicitly turning the feature of in a specific circumstance has a clear effect. Also, it could be useful for experimenting with the setting directly on the command line.

Anyway, thank you for the solution to my use case!

[gustavgransbo]

I would really appreciate some feedback in regards to this issue and my PR! @hauntsaninja