code: def Filter(self, match=None, **_):
      
      arg = self.stack.pop(-1)
  
      # Filters can be specified as a comma separated list.
      for filter_name in match.group(1).split(","):
        filter_object = ConfigFilter.classes_by_name.get(filter_name)
        if filter_object is None:
          raise FilterError("Unknown filter function %r" % filter_name)
  
        if not filter_object.sensitive_arg:
          logging.debug("Applying filter %s for %s.", filter_name, arg)
        arg = filter_object().Filter(arg)
        precondition.AssertType(arg, Text)
  
      self.stack[-1] += arg
docs: Filter the current expression.