let
  sources = import nix/sources.nix;
in
{ pkgs ? import sources.release2015 {}
, pypiData ? sources.pypi-deps-db
, mach-nix ? import sources.mach-nix { inherit pkgs pypiData; }
, tahoe-lafs ? "1.14.0"
}:
  let
    providers = {
      _default = "sdist,nixpkgs,wheel";
      # not packaged in nixpkgs at all, we can use the binary wheel from
      # pypi though.
      python-challenge-bypass-ristretto = "wheel";
      # Pure python packages that don't build correctly from sdists
      # - patches in nixpkgs that don't apply
      # - missing build dependencies
      platformdirs = "wheel";
      boltons = "wheel";
      klein = "wheel";
      humanize = "wheel";
      chardet = "wheel";
      urllib3 = "wheel";
      zipp = "wheel";
    };
    tahoe-versions = {
      "1.14.0" = {
        requirements = builtins.readFile ./requirements/base.txt;
        propagatedBuildInputs = [];
      };
      "1.16.0rc1" = rec {
        requirements = builtins.readFile ./requirements/base-1.16.txt;
        propagatedBuildInputs = [
          (
            mach-nix.buildPythonPackage {
              python = "python27";
              pname = "tahoe-lafs";
              version = "1.16.0rc1";
              inherit requirements providers;
              postPatch = ''
                cat > src/allmydata/_version.py <<EOF
                # This _version.py is generated by nix.

                verstr = "$version"
                __version__ = verstr
                EOF
                '';
              src = pkgs.fetchFromGitHub {
                owner = "fenn-cs";
                repo = "tahoe-lafs";
                rev = "f6a96ae3976ee21ad0376f7b6a22fc3d12110dce";
                sha256 = "ZN2V5vH+VqPaBmQXXqyH+vUiqW1YNhz+7LsiNNhA/4g=";
              };
            }
          )
        ];
      };
    };
  in
#    (builtins.elemAt tahoe-versions."1.16.0rc1".propagatedBuildInputs 0)
#/*
  mach-nix.buildPythonApplication rec {
    python = "python27";
    name = "zero-knowledge-access-pass-authorizer";
    src = ./.;
    inherit providers;
    inherit (tahoe-versions.${tahoe-lafs}) requirements propagatedBuildInputs;
    _.tahoe-lafs.patches = [
      (
        pkgs.fetchpatch {
          url = "https://raw.githubusercontent.com/PrivateStorageio/nixpkgs/privatestorageio/pkgs/development/python-modules/tahoe-lafs/rsa-exponent.patch";
          sha256 = "sha256-0vIMj5gZPbKLkow6wpA+Tz7bpyy+mZRSSFGmpg0VMyk=";
        }
      )
    ];
    format = "setuptools";
    # Record some settings here, so downstream nix files can consume them.
    meta.mach-nix = { inherit python providers; };
  }
  /**/