code: def expand_filename_pattern(self, pattern, base_dir, sourcefile=None):
          expandedPattern = substitute_vars([pattern], self, sourcefile)
          assert len(expandedPattern) == 1
          expandedPattern = expandedPattern[0]
          if expandedPattern != pattern:
              logging.debug("Expanded variables in expression %r to %r.",
                            pattern, expandedPattern)
          fileList = util.expand_filename_pattern(expandedPattern, base_dir)
          fileList.sort()
          if not fileList:
              logging.warning("No files found matching %r.", pattern)
          return fileList