qapi/schema: Don't initialize "members" with `None`

Declare, but don't initialize the "members" field with type
List[QAPISchemaObjectTypeMember].

This simplifies the typing from what would otherwise be
Optional[List[T]] to merely List[T]. This removes the need to add
assertions to several callsites that this value is not None - which it
never will be after the delayed initialization in check() anyway.

The type declaration without initialization trick will cause accidental
uses of this field prior to full initialization to raise an
AttributeError.

(Note that it is valid to have an empty members list, see the internal
q_empty object as an example. For this reason, we cannot use the empty
list as a replacement test for full initialization and instead rely on
the _checked/_check_complete fields.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240315152301.3621858-17-armbru@redhat.com>

scripts/qapi/schema.py

@@ -20,7 +20,7 @@ from abc import ABC, abstractmethod
 from collections import OrderedDict
 import os
 import re
-from typing import List, Optional
+from typing import List, Optional, cast
 
 from .common import (
     POINTER_SUFFIX,
@@ -449,7 +449,7 @@ class QAPISchemaObjectType(QAPISchemaType):
         self.base = None
         self.local_members = local_members
         self.variants = variants
-        self.members = None
+        self.members: List[QAPISchemaObjectTypeMember]
         self._check_complete = False
 
     def check(self, schema):
@@ -482,7 +482,11 @@ class QAPISchemaObjectType(QAPISchemaType):
         for m in self.local_members:
             m.check(schema)
             m.check_clash(self.info, seen)
-        members = seen.values()
+
+        # self.check_clash() works in terms of the supertype, but
+        # self.members is declared List[QAPISchemaObjectTypeMember].
+        # Cast down to the subtype.
+        members = cast(List[QAPISchemaObjectTypeMember], list(seen.values()))
 
         if self.variants:
             self.variants.check(schema, seen)
@@ -515,11 +519,9 @@ class QAPISchemaObjectType(QAPISchemaType):
         return self.name.startswith('q_')
 
     def is_empty(self):
-        assert self.members is not None
         return not self.members and not self.variants
 
     def has_conditional_members(self):
-        assert self.members is not None
         return any(m.ifcond.is_present() for m in self.members)
 
     def c_name(self):