docs: def add_custom_arg(self, custom_arg):
          """Add custom args to the email globaly or to a specific Personalization
  
          :param value: A CustomArg object or a dict of custom arg key/values
          :type value: CustomArg, dict
          """
          if custom_arg.personalization is not None:
              try:
                  personalization = \
                      self._personalizations[custom_arg.personalization]
                  has_internal_personalization = True
              except IndexError:
                  personalization = Personalization()
                  has_internal_personalization = False
              if isinstance(custom_arg, dict):
                  (k, v) = list(custom_arg.items())[0]
                  personalization.add_custom_arg(CustomArg(k, v))
              else:
                  personalization.add_custom_arg(custom_arg)
  
              if not has_internal_personalization:
                  self.add_personalization(
                      personalization, index=custom_arg.personalization)
          else:
              if isinstance(custom_arg, dict):
                  (k, v) = list(custom_arg.items())[0]
                  self._custom_args = self._ensure_append(
                      CustomArg(k, v), self._custom_args)
              else:
                  self._custom_args = self._ensure_append(
                      custom_arg, self._custom_args)