qapi/introspect.py: Type _gen_tree variants as Sequence[str]

Optional[List] is clunky; an empty sequence can more elegantly convey
"no variants". By downgrading "List" to "Sequence", we can also accept
tuples; this is useful for the empty tuple specifically, which we may
use as a default parameter because it is immutable.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210216021809.134886-18-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Doc string touched up]
Reviewed-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/introspect.py

@@ -249,13 +249,13 @@ def _use_type(self, typ: QAPISchemaType) -> str:
         return self._name(typ.name)
 
     @staticmethod
-    def _gen_features(features: List[QAPISchemaFeature]
+    def _gen_features(features: Sequence[QAPISchemaFeature]
                       ) -> List[Annotated[str]]:
         return [Annotated(f.name, f.ifcond) for f in features]
 
     def _gen_tree(self, name: str, mtype: str, obj: Dict[str, object],
                   ifcond: Sequence[str],
-                  features: Optional[List[QAPISchemaFeature]]) -> None:
+                  features: Sequence[QAPISchemaFeature] = ()) -> None:
         """
         Build and append a SchemaInfo object to self._trees.
 
@@ -302,7 +302,7 @@ def _gen_variant(self, variant: QAPISchemaVariant
 
     def visit_builtin_type(self, name: str, info: Optional[QAPISourceInfo],
                            json_type: str) -> None:
-        self._gen_tree(name, 'builtin', {'json-type': json_type}, [], None)
+        self._gen_tree(name, 'builtin', {'json-type': json_type}, [])
 
     def visit_enum_type(self, name: str, info: Optional[QAPISourceInfo],
                         ifcond: Sequence[str],
@@ -320,7 +320,7 @@ def visit_array_type(self, name: str, info: Optional[QAPISourceInfo],
                          element_type: QAPISchemaType) -> None:
         element = self._use_type(element_type)
         self._gen_tree('[' + element + ']', 'array', {'element-type': element},
-                       ifcond, None)
+                       ifcond)
 
     def visit_object_type_flat(self, name: str, info: Optional[QAPISourceInfo],
                                ifcond: Sequence[str],