Namespace packages supported by default?

[jaraco]

I notice in the changelog that 0.990 adds support for namespace packages by default.

I've been using mypy on namespace packages (jaraco, svg, backports, and others) for some time now (jaraco/skeleton@7455f2f).

With the release of mypy 0.990, however, tests have started failing across all projects with namespace packages:

error: Source file found twice under different module names: "collections" and "jaraco.collections"

The change log indicates that 0.990 adds support for namespace packages by default. Annoyingly, this change has had the opposite effect for my projects. I find that I can restore functionality by adding explicit_package_bases = True to the config file (or equivalent command line). It's slightly annoying to have to add this configuration to all of my projects just to support some projects with namespace packages.

Is it possible that explicit package bases will become default in the future?

[jaraco]

The reason why I ask is because in the current state, namespace packages get a second-class experience.

[hauntsaninja]

We chose not to make it the default at the time it was introduced because doing so was very breaking, e.g. all packages with src layout would probably break. It still remains true that most Python packages are not namespace packages, so it feels hard to justify.

(Not present in your report, but I believe the error message should currently suggest using --explicit-package-bases; let me know if that's not the case)

[jaraco]

Correct. The suggestion wasn’t apparent when running under pytest-mypy. Once I ran the tests using mypy directly, the suggestion was present and led me to the workaround.