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[RFC,18/32] python//qmp.py: add casts to JSON deserialization

Message ID 20200514055403.18902-19-jsnow@redhat.com
State New
Headers show
Series python/qemu: refactor as installable package | expand

Commit Message

John Snow May 14, 2020, 5:53 a.m. UTC
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>
---
 python/qemu/lib/qmp.py | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)
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Patch

diff --git a/python/qemu/lib/qmp.py b/python/qemu/lib/qmp.py
index 0036204218..e460234f2e 100644
--- a/python/qemu/lib/qmp.py
+++ b/python/qemu/lib/qmp.py
@@ -13,6 +13,7 @@ 
 import logging
 from typing import (
     Any,
+    cast,
     Dict,
     Optional,
     TextIO,
@@ -129,7 +130,10 @@  def __json_read(self, only_event=False):
             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 = cast(QMPMessage, json.loads(data))
             if 'event' in resp:
                 self.logger.debug("<<< %s", resp)
                 self.__events.append(resp)
@@ -261,7 +265,7 @@  def command(self, cmd, **kwds):
         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):
         """