qapi/expr.py: constrain incoming expression types

mypy does not know the types of values stored in Dicts that masquerade
as objects. Help the type checker out by constraining the type.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210421182032.3521476-5-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/expr.py

@@ -15,9 +15,20 @@
 # See the COPYING file in the top-level directory.
 
 import re
+from typing import Dict, Optional
 
 from .common import c_name
 from .error import QAPISemError
+from .parser import QAPIDoc
+from .source import QAPISourceInfo
+
+
+# Deserialized JSON objects as returned by the parser.
+# The values of this mapping are not necessary to exhaustively type
+# here (and also not practical as long as mypy lacks recursive
+# types), because the purpose of this module is to interrogate that
+# type.
+_JSONObject = Dict[str, object]
 
 
 # Names consist of letters, digits, -, and _, starting with a letter.
@@ -315,9 +326,20 @@ def check_event(expr, info):
 
 def check_exprs(exprs):
     for expr_elem in exprs:
-        expr = expr_elem['expr']
-        info = expr_elem['info']
-        doc = expr_elem.get('doc')
+        # Expression
+        assert isinstance(expr_elem['expr'], dict)
+        for key in expr_elem['expr'].keys():
+            assert isinstance(key, str)
+        expr: _JSONObject = expr_elem['expr']
+
+        # QAPISourceInfo
+        assert isinstance(expr_elem['info'], QAPISourceInfo)
+        info: QAPISourceInfo = expr_elem['info']
+
+        # Optional[QAPIDoc]
+        tmp = expr_elem.get('doc')
+        assert tmp is None or isinstance(tmp, QAPIDoc)
+        doc: Optional[QAPIDoc] = tmp
 
         if 'include' in expr:
             continue