docs: def _file_lines(self, filename):
          """Get lines for filename, caching opened files."""
          try:
              return self._file_lines_cache[filename]
          except KeyError:
              if os.path.isfile(filename):
                  with open(filename) as python_file:
                      self._file_lines_cache[filename] = python_file.readlines()
              else:
                  self._file_lines_cache[filename] = ""
  
              return self._file_lines_cache[filename]