anchor.diff:  
     pychart = buildPythonPackage rec {
       name = "pychart-1.39";
  +    disabled = ! isPy27;
   
       src = pkgs.fetchurl {
         url = "http://download.gna.org/pychart/PyChart-1.39.tar.gz";
  
positive.code: 
    datadiff = buildPythonPackage rec {
      name = "datadiff-1.1.6";
  
      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/source/d/datadiff/datadiff-1.1.6.zip";
  
negative.code: 
    html2text = buildPythonPackage rec {
      name = "html2text-2014.12.29";
      disabled = ! isPy27;
  
      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/source/h/html2text/html2text-2014.12.29.tar.gz";
  