code: def _debug(self, out, print_prefix=True):
          
          if self.debug:
              if print_prefix:
                  pre = self.__class__.__name__
                  if hasattr(self, 'debug_prefix'):
                      pre = getattr(self, 'debug_prefix')
                  sys.stderr.write("%s: " % pre)
              sys.stderr.write(out)
docs: Print out to stderr, if debugging is enabled.