[CaliforniaKarl]

This, I think, is the crux of the matter.

As an example, I always knew urllib3 as one of the foundational packages that Requests uses. And I was curious, what versions of urllib3 does Requests pull in?

Well, according to https://github.com/psf/requests/blob/main/setup.cfg, it's this:

    urllib3>=1.21.1,<3

That is exactly the kind of dependency specification I would expect to see for a package that is using semver: The current version of urllib3 is 2.x, so with semver, you set up your dependencies to avoid the next major-version number (in this case, 3).

So, it seems to me that even the Requests folks assumed urllib3 was using semver.

[somat]

I would almost expect the 3 in urllib3 to be the major version and if something needed to break it would become urllib4. Which, I know, is terribly naive of me. But that is how psycopg does it.

[minitech]

That was how psycopg2 did it, but now the package is psycopg (again) version 3, as it should be. Python package management has come a long way since psycopg 1 was created.

urllib2/3’s etymology is different: urllib2’s name comes from urllib in the standard library.