How should pip handle extras?

Hey, all. I previously ran into what I thought was a bug with pip’s handling of package extras. When you install a package with extras, pip doesn’t remember the dependency relations of the extras after installation. I initially filed an issue with pip, which has since been closed. Here’s the gist:

As an example, create example-package with an extra dependency. Using setuptools, the setup.cfg file would look something like this:

name = example-package
version = 1.0
install_requires =
  psycopg[binary] >=3.0

Here, psycopg[binary] adds a psycopg-binary package dependency.

Then, install example-package. It will install psycopg-binary, psycopg, and example-package. The first package here is an extra dependency.

Now, pip show example-package will list psycopg as example-package's only requirement. Additionally, pip show psycopg-binary will state that psycopg-binary is not required by any other package. If you pip uninstall psycopg-binary, pip check will not state that there are broken dependencies.

I was under the impression that all dependencies fall into one of two categories: build dependencies (specified in a pyproject.toml file per PEP 517/518) and everything else (tool-dependent, but typically specified in a file using setuptools). Build dependencies need not persist after build-time, and all other dependencies persist as long as the packages requiring them are installed. In my example, example-project's metadata states that it requires psycopg[binary]. After initial installation, this metadata persists; the package still has this requirement.

What are people’s thoughts on this? PEP 508 states how to specify package dependencies. Is there a similar PEP that states what dependencies really “mean”? If not, how do you all think that extras should be handled?