Presence of a plugin causes doctests a shared namespace to fail traceback with assertion

In jaraco/jaraco.test#3, I've described an issue that appears to implicate some of the most edgy features of pytest and Python, in particular:

• assert rewrite
• doctests
• namespace packages
• plugins

Oh, joy.

In summary, the issue seems to be that:

The presence of an empty module as a plugin in a namespace shared by the package under test causes assert exceptions in doctests to be re-written, causing the output from those doctests to fail where they wouldn't otherwise, even if the plugin is disabled with -p no:....

As you can see from the tox execution in the downstream report, I'm using pytest 6.1.2 on Python 3.9.0 with jaraco.test 4.0.1 (the implicated plugin) on macOS 11.0.1 (latest stable everything as of this writing).

I don't yet have a minimal example, though you can check out jaraco/jaraco.itertools@e9f23a4 and run tox -- -k assert to see the error.

I expect (though haven't verified) that any of the following remedies might be possible:

• Move the plugin to a package not sharing the jaraco namespace. I was considering creating pytest-enabler anyway.
• Replace the bare assert with a raise AssertionError. I'll test that soon.
• Update the test expectation to expect the rewritten assert output.
• Replace the doctest with a unit test.

None of these workarounds would address the root cause, but instead avoid the confounding factors leading to the failed expectation.

I recognize this issue is probably too obscure to expect it to be fixed upstream, but I could really use some advice on how to proceed with an investigation. In particular, can you explain or point me to the implementation where doctests get special treatment for assert-rewrite (or vice-versa), especially any code that might be relevant for selectively affecting the modules-under-test or plugins?

Start by checking out jaraco/jaraco.itertools at e9f23a4 and run tox -- -k assert to reproduce the doctest failure. Then trace pytest's doctest handling and assertion rewriting, focusing on the effect of an empty plugin module in the shared jaraco namespace and whether disabling the plugin changes it. Done means documenting a minimal reproduction and a clear root cause or upstream direction.