docs: def _register_custom_filters(self):
          """Register any custom filter modules."""
          custom_filters = self.settings.get('CUSTOM_FILTERS', [])
          if not isinstance(custom_filters, list):
              raise KeyError("`CUSTOM_FILTERS` setting must be a list.")
  
          for filter_module_name in custom_filters:
              try:
                  filter_module = import_module(filter_module_name)
              except ImportError as error:
                  raise ImproperlyConfigured(
                      "Failed to load custom filter module '{}'.\n"
                      "Error was: {}".format(filter_module_name, error)
                  )
  
              try:
                  filter_map = getattr(filter_module, 'filters')
                  if not isinstance(filter_map, dict):
                      raise TypeError
              except (AttributeError, TypeError):
                  raise ImproperlyConfigured(
                      "Filter module '{}' does not define a 'filters' dictionary".format(filter_module_name)
                  )
              self._environment.filters.update(filter_map)