Merge pull request #6482 from ViktorHaag/object-descr-setting-class-t…

…ype-map-iss6478

Explicitly treat ObjectDescription._doc_field_type_map as an instance variable

sphinx/directives/__init__.py

@@ -73,6 +73,7 @@ class ObjectDescription(SphinxDirective):
     def get_field_type_map(self):
         # type: () -> Dict[str, Tuple[Field, bool]]
         if self._doc_field_type_map == {}:
+            self._doc_field_type_map = {}
             for field in self.doc_field_types:
                 for name in field.names:
                     self._doc_field_type_map[name] = (field, False)