How does nix solves the problem of Python dependencies not specifying their dependencies well or exhaustively? Do you have to find them out and fix them manually or can you generate a lock file and hope for the best?
nixpkgs maintainers are frequently the first to notify a project author of incompatibility with new versions of another package.
In Nix lingo, a package successfully building or running against an implicit or unmanaged dependency is called 'impurity'. To help keep builds 'pure', Nix builds everything in a sandbox and does various other tricks to ensure that a package being built can't/doesn't find any dependencies at build time that you don't explicitly tell Nix to include in the build environment.
(This is also increasingly how Linux distros build their packages, to solve the same problem.)
For the most part, if building (and running the test suite during the build) a Python package with Nix succeeds, you can be confident that you've got all the dependencies sorted out— even the ones upstream forgot to tell you about.
> Do you have to find them out and fix them manually or can you generate a lock file and hope for the best?
At install time, you can be confident that Nix will bring along all of the system-level dependencies your Python package needs. You don't have to find and fill any gaps at install time 10 years from now or whatever.
When you're writing your Nix package for the first time, you'll be doing a mix of generating the Nix code that defines your package from some upstream, Python-specific lockfile and making manual corrections when the build fails. Nix doesn't have any magic for figuring out dependencies that are left out of poetry.lock or whatever, or for disambiguating guaranteed-compatible exact versions from requirements.txt.