I use a default.nix file and a requirements.txt file and then with a single command I'm into a shell and virtual environment with all dependencies and packages installed, that I can easily transfer between machines.
That is unless I want to use PyQt5.
A purely functional package manager, distro, devops. And pretty soon, home directory management. Maintaining servers or doing aggressive changes becomes very easy. There's even a Darwin (macOS) implementation now, so you can manage most of your Mac functionally (with heavy usage of defaults under the hood).
It's still lacking a bit on usability, as some things are not as intuitive as in a simple imperative distribution such as Arch or Alpine. Especially if you need to run some prepackaged software that assumes FHS and binds dynamically to some pre-existing dependencies.
But to me, right now if you are a moderately advanced user I don't see a point in running distros that are stuck in the middle (imperative, complex, lots of defaults). It's either one extreme (simple and imperative, e.g. Arch) or the other (functional, NixOS).
In Python (or basically any other language package manager) you can run arbitrary scripts (post-install etc) and so this doesn't lend itself nicely to a reproducible, functional approach.
Then the result of building a particular package is installed in /nix/store/hash-packagename. And this package links to other packages in the nix store using precise hashes. There is no dynamic linking. So the result is referentially transparent. A particular hash is guaranteed to correspond to the same package version, built in the same way and linked to the same dependencies. Furthermore, installing new package versions or modified versions of a package wont overwrite old ones, as hashes are different.
The same concept applies to a whole system setup, which is identified by a hash computed using all options that configure the system plus all packages available in your environment.
On the surface from what I've seen it does all the things right and it is kind of what I expected Docker would before I used it.
People do though.
It's actually pyqtchart and qscintilla that are the problems. They're not packaged for nix and won't install in a virtualenv with pip. Something to do with hardcoded paths for dependencies I think.
I'm going to have a go at writing a nix files that build them from source, with the help of a NixOS expert, so we'll see how that goes.