broken.nix

with import <nixpkgs> {};
with pkgs.python3Packages;

stdenv.mkDerivation {
  name = "impurePythonEnv";
  buildInputs = [
    python3Full
    pipenv.overrideAttrs ( oldAttrs: {
      version = "master";

      src = fetchFromGitHub {
        owner = "pypa";
        repo = "pipenv";
        rev = "45c337a40f050ab6654346152f4f711e6566027d";
        sha256 = "1jv9y3kn6ki4p9sb45986h8jhrvvagi5b57mrh4271fz63m8y83g";
      };
    })
    
    psutil
    stdenv
  ];

  src = null;
  shellHook = ''
  # set SOURCE_DATE_EPOCH so that we can use python wheels
  SOURCE_DATE_EPOCH=$(date +%s)
  pipenv shell
  '';
}

gistfile1.txt

$ nix-shell broken.nix --show-trace
error: while evaluating the attribute 'buildInputs' of the derivation 'impurePythonEnv' at /nix/store/hncg8ii3iv6ldlk55kvyzmic7b40zpgn-nixpkgs-18.09pre146409.2391d1dbf31/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:175:11:
cannot coerce a function to a string, at /nix/store/hncg8ii3iv6ldlk55kvyzmic7b40zpgn-nixpkgs-18.09pre146409.2391d1dbf31/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:175:11

working.nix

with import <nixpkgs> {};
with pkgs.python3Packages;

stdenv.mkDerivation {
  name = "impurePythonEnv";
  buildInputs = let
    pipenv-master = pipenv.overrideAttrs ( oldAttrs: {
      version = "master";

      src = fetchFromGitHub {
        owner = "pypa";
        repo = "pipenv";
        rev = "45c337a40f050ab6654346152f4f711e6566027d";
        sha256 = "1jv9y3kn6ki4p9sb45986h8jhrvvagi5b57mrh4271fz63m8y83g";
      };
    });
  in [
    python3Full
    pipenv-master
    
    psutil
    stdenv
  ];

  src = null;
  shellHook = ''
  # set SOURCE_DATE_EPOCH so that we can use python wheels
  SOURCE_DATE_EPOCH=$(date +%s)
  pipenv shell
  '';
}