It depends. If the dependency is in nixpkgs (Nix' own package repository), someone will have done the work of figuring out the dependencies properly and you can just use that. If it's not, you can either do that yourself and pin particular versions, or you can integrate with a tool like Poetry: Poetry could generate a lockfile for you that you then reference from Nix to get the versions of Python packages. You'd still need to specify any native dependencies manually, though.

Ultimately the true bellwether of compatibility is the package's own test suite, which we try to get working (and integrated into the build process) for as many packages as possible. For packages which have poor/non-existent test suites, often a downstream package's test suite will expose compatibility problems (we've found bugs in zlib security patches using curl's test suite for instance).

nixpkgs maintainers are frequently the first to notify a project author of incompatibility with new versions of another package.

> How does nix solves the problem of Python dependencies not specifying their dependencies well or exhaustively?

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.