python # predefined undocumented variables? How/where to find those?

[InLaw]

falke.nix

{
  # This example flake.nix is pretty generic and the same for all
  # examples, except when they define devShells or extra packages.
  description = "d2n Dream2nix example flake";

  # We import the latest commit of dream2nix main branch and instruct nix to
  # re-use the nixpkgs revision referenced by dream2nix.
  # This is what we test in CI with, but you can generally refer to any
  # recent nixpkgs commit here.
  inputs = {
    dream2nix.url = "github:nix-community/dream2nix";
    nixpkgs.follows = "dream2nix/nixpkgs";
  };

  outputs = {
    self,
    dream2nix,
    nixpkgs,
  }: 
  let
    # A helper that helps us define the attributes below for
    # all systems we care about.
    eachSystem = nixpkgs.lib.genAttrs [
      "aarch64-darwin"
      "aarch64-linux"
      "x86_64-darwin"
      "x86_64-linux"
    ];

    pkgs = import nixpkgs {
        system = "x86_64-linux";
      };

  in 
  {
    packages = eachSystem (system: {
      # For each system, we define our default package
      # by passing in our desired nixpkgs revision plus
      # any dream2nix modules needed by it.
      default = dream2nix.lib.evalModules {
        packageSets.nixpkgs = nixpkgs.legacyPackages.${system};

        modules = [
          # Import our actual package definiton as a dream2nix module from ./default.nix
          ./default.nix 
          {
            # Aid dream2nix to find the project root. This setup should also works for mono
            # repos. If you only have a single project, the defaults should be good enough.
            paths.projectRoot = ./.;
            # can be changed to ".git" or "flake.nix" to get rid of .project-root
            paths.projectRootFile = "flake.nix";
            paths.package = ./.;
            }
        ];
      };

    });

    devShells = eachSystem (system: {
      default = nixpkgs.legacyPackages.${system}.mkShell {
        # inherit from the dream2nix generated dev shell
        inputsFrom = [self.packages.${system}.default.devShell];
        # add extra packages
        packages = ( with nixpkgs.legacyPackages.${system} ; [
          python.pkgs.ipython
        ] ) ;
      };

    });
  };


}

python[.pkgs.ipython] is not defined before? The code should fail, no?
The code works and python is

• error: Package ‘python-2.7.18.8’ in /nix/store/jyw83fn5spbdgzdj110620dm0zl43q2y-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:341 is marked as insecure, refusing to evaluate.

default.nix

{
  config,
  lib,
  dream2nix,
  ...
}: 
let
  
in
{
  imports = [
    dream2nix.modules.dream2nix.WIP-python-pdm
  ];

  mkDerivation = {
    src = lib.cleanSourceWith { 
      src = lib.cleanSource ./.;
      filter = name: type:
        !(builtins.any (x: x) [
          (lib.hasSuffix ".nix" name)
          (lib.hasPrefix "." (builtins.baseNameOf name))
          (lib.hasSuffix "flake.lock" name)
        ]);
    };

  };
  pdm.lockfile = ./pdm.lock;
  pdm.pyproject = ./pyproject.toml; 

  buildPythonPackage = {
    pythonImportsCheck = [
      "my_tool"
    ];
  };
}

Where is python 2.7 defined?
How to show all other already defined variables?