code: def input_file(filename):
      
      if excluded(filename) or not filename_match(filename):
          return {}
      if options.verbose:
          message('checking ' + filename)
      options.counters['files'] = options.counters.get('files', 0) + 1
      errors = Checker(filename).check_all()
      if options.testsuite and not errors:
          message("%s: %s" % (filename, "no errors found"))
      return errors
docs: Run all checks on a Python source file.