Development environments using nix-shell no longer work

[jaraco]

The change in #2165 broke development environments using nix-shell, which offers a feature similar to running setup.py develop. If I manually copy the site.py generated by older versions of setuptools into the new environment I can fix it, but this is clearly a hack.

I'm not sure how to proceed to properly fix this issue when using nix-shell. @jaraco would you be able to help recommending workarounds to solve this problem?

Originally posted by @cript0nauta in #2165 (comment)

[jaraco]

In the referenced commit, I've created a test that captures the reported failure. I'm still not certain it's a use-case that Setuptools should support, but it at least provides a means to evaluate more easily.

[jaraco]

In this latest commit, in the test, I illustrate how it's possible to inject a sitecustomize.py into the target site-packages directory (a much simpler solution than the site.py hack from before).

My instinct here is it would be best for nix to own this behavior, rather than Setuptools.

@limeytexan or @cript0nauta, would you comment on how feasible it would be for nix to adopt the technique in the illustrative test? Such a workaround would provide compatibility for existing builds of Setuptools as well.

[cript0nauta]

Thanks for proposing a solution! I made a proof of concept that adds a similar sitecustomize.py file in Nix Python environments and it seems to work ok now. I'll check it and open a PR in nixpkgs (so there would be no need to update setuptools).

[jaraco]

That’s great news. In that case, I’ll merge the related Setuptools PR with the test. Keep us apprised here with the response in Nix.