I still have no idea how it all works but it seemed prudent for me to at least try.
It's better to instead pip-install Python packages into virtual environments, recent Pythons havr `venv` built in for this purpose. For user-scoped or system-scoped utilities, `pipx` can manage dedicated virtual environments and symlink them into the search path.
I think this is insane. Not only will many packages be missing from Nix, you will also have to wait for the upstream changes to actually propagate to Nix repositories. This all assumes, of course, that there are no packaging issues or incompatibilities in this repackaging.
This is one of the ways that Nix sometimes just gets in your way. I've been using Nix(OS) for several years now, and this still bothers me.
Instead of doing this, For Python specifically I would suggest installing pyenv, which Nix packages. Then enter a nix-shell with a derivation thingie[1,2], and install Python as usual with pyenv. Then you can use any Python version, and with pyenv-virtualenv (which Nix _doesn't_ package...), you can use venvs as you're used to. Sure, you don't get the benefits of the declarative approach and isolation as with "the Nix way", and you may run into other issues, but at least it's a workflow well known to Python devs. Hope it helps!
[1]: https://gist.github.com/imiric/3422258c4df9dacb4128ff94d31e9...
[2]: It took me way longer than I would like to admit to figure this out... This shouldn't be so difficult!
nix-shell -p 'python3.withPackages(ps: [ ps.numpy ])'
nix-shell -p python3 python3Packages.numpy
The advantage of this approach is that it gives more control to the distro maintainers and the admins of the computer, taking that control away from the "upstream" maintainers of the software being packaged. For example the software being packaged cannot just call the library bar because bar is not at /usr/lib/bar.so like it is in most Linux distros -- it is at /nix/store/17813e8b97b84e0317813e8b97b84e03-bar/usr/lib/bar.so, but of course the software does not know that unless the person creating the Nix package (the packager) arranges for the software to know it (again sometimes by doing a search-and-replace on binaries).
Note that if this separation of packages imposed by NixOS has any beneficial security properties, it is merely security through obscurity because there is nothing preventing a binary from covertly searching through the directory listing of /nix/store/ for the name of the library it wants to call. Nevertheless it turns out the be useful to seize some control away from upstream in this way even if technically upstream could seize the control back if it were willing to complicate the software to do so.
People, including the creator of Nix and NixOS (Dolstra), will tell you that NixOS's main advantage is "declarativeness" (which in the past Dolstra called "purity") or the fact that the compilation / building process is deterministic. I believe they are mistaken about that. Specifically, I believe that being able to rely on the build process of a package to be deterministic is useful, the separation I described is much more useful to most users and prospective users of NixOS.