Python doesn't have anything quite exactly like Traits, Interfaces, or
Mixins; but we can approximate it.

Add a 'Visitable' class that enforces a type signature for the visit()
method; this way, mypy is ensuring that even for otherwise unrelated
classes that do not inherit from a common base, this signature will
always be forced to be compatible.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 scripts/qapi/schema.py | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
index 51af0449f5..55434f5c82 100644
--- a/scripts/qapi/schema.py
+++ b/scripts/qapi/schema.py
@@ -25,7 +25,13 @@
 from .parser import QAPISchemaParser
 
 
-class QAPISchemaEntity:
+class Visitable:
+    """Abstract duck that suggests a class is visitable."""
+    def visit(self, visitor: 'QAPISchemaVisitor') -> None:
+        raise NotImplementedError
+
+
+class QAPISchemaEntity(Visitable):
     meta: Optional[str] = None
 
     def __init__(self, name: str, info, doc, ifcond=None, features=None):
@@ -136,7 +142,7 @@ def visit_event(self, name, info, ifcond, features, arg_type, boxed):
         pass
 
 
-class QAPISchemaModule:
+class QAPISchemaModule(Visitable):
     def __init__(self, name):
         self.name = name
         self._entity_list = []
@@ -812,7 +818,7 @@ def visit(self, visitor):
             self.arg_type, self.boxed)
 
 
-class QAPISchema:
+class QAPISchema(Visitable):
     def __init__(self, fname):
         self.fname = fname
         parser = QAPISchemaParser(fname)
-- 
2.26.2