Re: [PATCH v2 05/16] python/qmp.py: add casts to JSON deserialization

[Kevin Wolf <kwolf@redhat.com>]

Am 02.06.2020 um 23:45 hat John Snow geschrieben:
> 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/qmp.py | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/python/qemu/qmp.py b/python/qemu/qmp.py
> index ef3c919b76c..5f3558e3066 100644
> --- a/python/qemu/qmp.py
> +++ b/python/qemu/qmp.py
> @@ -13,6 +13,7 @@
>  import logging
>  from typing import (
>      Any,
> +    cast,
>      Dict,
>      Optional,
>      TextIO,
> @@ -130,7 +131,10 @@ 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))

Instead of casting, you can just specify the variable type:

    resp: QMPMessage = json.loads(data)

I don't think that json.loads() will actually return something other
than Any anytime soon, but it's generally nicer to avoid casts and if it
eventually does change, we'll get the type check instead of silencing
it.

>              if 'event' in resp:
>                  self.logger.debug("<<< %s", resp)
>                  self.__events.append(resp)
> @@ -262,7 +266,7 @@ 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'])

This one can't be easily avoided, though.

Kevin