Explicitly treat ObjectDescription._doc_field_type_map as an instance variable

sphinx/directives/__init__.py

@@ -70,18 +70,25 @@ class ObjectDescription(SphinxDirective):
     # Warning: this might be removed in future version. Don't touch this from extensions.
     _doc_field_type_map = {}  # type: Dict[str, Tuple[Field, bool]]
 
+    def _process_type_map(self, typelist):
+        # type: (List[Field]) -> Dict[str, Tuple[Field, bool]]
+        typemap = {}
+        for field in typelist:
+            for name in field.names:
+                typemap[name] = (field, False)
+            if field.is_typed:
+                typed_field = cast(TypedField, field)
+                for name in typed_field.typenames:
+                    typemap[name] = (field, True)
+        return typemap
+
+    def __init__(self, *args, **kwargs):
+        # type: (*Any, **Any) -> None
+        SphinxDirective.__init__(self, *args, **kwargs)
+        self._doc_field_type_map = self._process_type_map(self.doc_field_types)
+
     def get_field_type_map(self):
         # type: () -> Dict[str, Tuple[Field, bool]]
-        if 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)
-
-                if field.is_typed:
-                    typed_field = cast(TypedField, field)
-                    for name in typed_field.typenames:
-                        self._doc_field_type_map[name] = (field, True)
-
         return self._doc_field_type_map
 
     def get_signatures(self):