[PATCH 10/19] qapi/schema: make QAPISchemaArrayType.element_type non-Optional

[John Snow <jsnow@redhat.com>]

This field should always be present and defined. Change this to be a
runtime @property that can emit an error if it's called prior to
check().

This helps simplify typing by avoiding the need to interrogate the value
for None at multiple callsites.

RFC: Yes, this is a slightly different technique than the one I used for
QAPISchemaObjectTypeMember.type; I think I prefer this one as being a
little less hokey, but it is more SLOC. Dealer's choice for which style
wins out -- now you have an example of both.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 scripts/qapi/schema.py | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
index c9a194103e1..462acb2bb61 100644
--- a/scripts/qapi/schema.py
+++ b/scripts/qapi/schema.py
@@ -366,7 +366,16 @@ def __init__(self, name, info, element_type):
         super().__init__(name, info, None)
         assert isinstance(element_type, str)
         self._element_type_name = element_type
-        self.element_type = None
+        self._element_type: Optional[QAPISchemaType] = None
+
+    @property
+    def element_type(self) -> QAPISchemaType:
+        if self._element_type is None:
+            raise RuntimeError(
+                "QAPISchemaArray has no element_type until "
+                "after check() has been run."
+            )
+        return self._element_type
 
     def need_has_if_optional(self):
         # When FOO is an array, we still need has_FOO to distinguish
@@ -375,7 +384,7 @@ def need_has_if_optional(self):
 
     def check(self, schema):
         super().check(schema)
-        self.element_type = schema.resolve_type(
+        self._element_type = schema.resolve_type(
             self._element_type_name, self.info,
             self.info and self.info.defn_meta)
         assert not isinstance(self.element_type, QAPISchemaArrayType)
-- 
2.41.0