anchor.diff:    pypi2nix = self.buildPythonPackage rec {
       rev = "04a68d8577acbceb88bdf51b1231a9dbdead7003";
       name = "pypi2nix-1.0_${rev}";
  +    disabled = ! isPy27;
   
       src = pkgs.fetchurl {
         url = "https://github.com/garbas/pypi2nix/tarball/${rev}";
  
positive.code: 
    basemap = buildPythonPackage rec {
      name = "basemap-1.0.7";
  
      src = pkgs.fetchurl {
        url = "mirror://sourceforge/project/matplotlib/matplotlib-toolkits/basemap-1.0.7/basemap-1.0.7.tar.gz";
  
negative.code:     version  = "1.4";
      baseName = "pync";
      name     = "${baseName}-${version}";
      disabled = ! isPy27;
  
      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/source/p/${baseName}/${name}.tar.gz";
  