anchor.diff:  
       src = pkgs.fetchurl {
         url = "http://pypi.python.org/packages/source/d/django-classy-tags/${name}.tar.gz";
  -      md5 = "eb686aa767ad8cf88c1fa0f400a42516";
         sha256 = "0wxvpmjdzk0aajk33y4himn3wqjx7k0aqlka9j8ay3yfav78bdq0";
       };
   
  
positive.code: 
      src = pkgs.fetchurl {
        url = "https://pypi.python.org/packages/source/p/python-bugzilla/python-${name}.tar.gz";
        # md5 = "c95befd1fecad21f742beaa8180538c0"; # provided by pypi website.
        sha256 = "11361635a4f1613803a0b9b93ba9126f7fd36180653f953e2590b1536d107d46";
      };
  
  
negative.code: 
      src = pkgs.fetchurl {
        url    = "https://pypi.python.org/packages/source/T/Twiggy/Twiggy-0.4.5.tar.gz";
        sha256 = "4e8f1894e5aee522db6cb245ccbfde3c5d1aa08d31330c7e3af783b0e66eec23";
      };
  
  