code: def _list_checkers(self):
          
          classes = set()
          for checker_group in checkers.checkers.itervalues():
              for checker in checker_group:
                  classes.add(checker)
  
          max_width = 0
          for clazz in classes:
              max_width = max(max_width, len(clazz.tool), len(clazz.__name__))
  
          for clazz in sorted(classes):
              status, _ = commands.getstatusoutput('which %s' % clazz.tool)
              result = 'missing' if status else 'installed'
              version = '' if status else clazz.get_version()
              print '%s%s%s%s' % (
                  clazz.__name__.ljust(max_width + 1),
                  clazz.tool.ljust(max_width + 1),
                  result.ljust(max_width + 1),
                  version,
              )
docs: Print information about checkers and their external tools.
  
          Currently only works properly on systems with the `which` tool
          available.