bar.nix

{ buildPythonPackage
, foo
}:

buildPythonPackage {
  pname = "bar";
  version = "1.0.0";
  src = ./bar;
  doCheck = false;
  pythonImportsCheck = ["bar"];
  nativeBuildInputs = [foo];
}

bar.py

from foo import foo
def bar(name):
    return f"bar {foo(name)}"

default.nix

{ pkgs ? import <nixpkgs> {} }:

let 
  packageOverrides = self: super: {
    foo = self.callPackage ./foo.nix {};
    bar = self.callPackage ./bar.nix {};
  };
  pythonWithMyPackages = (pkgs.python3.override { inherit packageOverrides; }).withPackages (ps:
    [
      ps.foo
      ps.bar
    ]
  );
in
  pythonWithMyPackages

foo.nix

{ buildPythonPackage
}:

buildPythonPackage {
  pname = "foo";
  version = "1.0.0";
  src = ./foo;
  pythonImportsCheck = ["foo"];
  doCheck = false;
}

foo.py

def foo(name):
    return f"foo {name}"

setup.py

from setuptools import setup

setup(
    name="bar",
    version="1.0.0",
    py_modules=["bar"],
    install_requires=[
        "foo>=1.0.0",
    ],
)

e.g.

git clone https://gist.github.com/rgoulter/c9536efea5a76dd30fd64c3daa5765d5 nix-python-packages
cd nix-python-packages
nix-build
cd ..
./nix-python-packages/result/bin/python

>>> from bar import bar
>>> bar("x")
'bar foo x'
>>>