python/qmp.py: add casts to JSON deserialization

mypy and python type hints are not powerful enough to properly describe
JSON messages in Python 3.6. The best we can do, generally, is describe
them as Dict[str, Any].

Add casts to coerce this type for static analysis; but do NOT enforce
this type at runtime in any way.

Note: Python 3.8 adds a TypedDict construct which allows for the
description of more arbitrary Dictionary shapes. There is a third-party
module, "Pydantic", which is compatible with 3.6 that can be used
instead of the JSON library that parses JSON messages to fully-typed
Python objects, and may be preferable in some cases.

(That is well beyond the scope of this commit or series.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200710052220.3306-6-jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

python/qemu/qmp.py

@@ -13,6 +13,7 @@ import socket
 import logging
 from typing import (
     Any,
+    cast,
     Dict,
     Optional,
     TextIO,
@@ -130,7 +131,10 @@ class QEMUMonitorProtocol:
             data = self.__sockfile.readline()
             if not data:
                 return None
-            resp = json.loads(data)
+            # By definition, any JSON received from QMP is a QMPMessage,
+            # and we are asserting only at static analysis time that it
+            # has a particular shape.
+            resp: QMPMessage = json.loads(data)
             if 'event' in resp:
                 self.logger.debug("<<< %s", resp)
                 self.__events.append(resp)
@@ -262,7 +266,7 @@ class QEMUMonitorProtocol:
         ret = self.cmd(cmd, kwds)
         if 'error' in ret:
             raise QMPResponseError(ret)
-        return ret['return']
+        return cast(QMPReturnValue, ret['return'])
 
     def pull_event(self, wait=False):
         """