code: def lint_files(self, files):
          
          all_violations = []
          for filename in files:
              with open(filename, 'r') as f:
                  content = f.read()
                  violations = self.lint(content)
                  all_violations.extend(violations)
                  for e in violations:
                      print("{0}:{1}: {2} {3}".format(filename, e.line_nr, e.rule_id, e.message))
          return len(all_violations)
docs: Lints a list of files.
          :param files: list of files to lint
          :return: a list of violations found in the files