code: def get_filenames_diff(self, r):
          
          cmd = self.filenames_diff_cmd(r)
  
          diff_files = shell_out_ignore_exitcode(cmd, cwd=self.root)
          diff_files = self.parse_diff_filenames(diff_files)
  
          return set(f for f in diff_files if f.endswith('.py'))
docs: Get the py files which have been changed since rev.