docs: def is_valid(self, attrs = None):
          """
          Validate an object against the __validates__ class variable.
          Returns True if valid, and False if any validations fail. If passed
          (as a dict), +attrs+ will be populated with the invalid attributes.
          """
          if attrs is None:
              reason = {}
          else:
              reason = attrs
          valid = True
          for attr, validation in self.__class__.__validates__.items():
              if validation.__class__ == validators.validation:
                  validation.name = attr
              if not validation(self):
                  reason[attr] = getattr(self, attr)
                  valid = False
          return valid