Re: [PATCH RFC 4/7] message: add QMP Message type

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Apr 13, 2021 at 11:55:50AM -0400, John Snow wrote:
> This is an abstraction that represents a single message either sent to
> or received from the server. It is used to subclass the
> AsyncProtocol(Generic[T]) type.
> 
> It was written such that it can be populated by either raw data or by a
> dict, with the other form being generated on-demand, as-needed.
> 
> It behaves almost exactly like a dict, but has some extra methods and a
> special constructor. (It should quack fairly convincingly.)
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  message.py | 196 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 196 insertions(+)
>  create mode 100644 message.py
> 
> diff --git a/message.py b/message.py
> new file mode 100644
> index 0000000..5c7e828
> --- /dev/null
> +++ b/message.py
> @@ -0,0 +1,196 @@
> +"""
> +QMP Message format and errors.
> +
> +This module provides the `Message` class, which represents a single QMP
> +message sent to or from the server. Several error-classes that depend on
> +knowing the format of this message are also included here.
> +"""
> +
> +import json
> +from json import JSONDecodeError
> +from typing import (
> +    Dict,
> +    ItemsView,
> +    Iterable,
> +    KeysView,
> +    Optional,
> +    Union,
> +    ValuesView,
> +)
> +
> +from error import (
> +    DeserializationError,
> +    ProtocolError,
> +    UnexpectedTypeError,
> +)
> +
> +
> +class Message:
> +    """
> +    Represents a single QMP protocol message.
> +
> +    QMP uses JSON objects as its basic communicative unit; so this
> +    object behaves like a MutableMapping. It may be instantiated from
> +    either another mapping (like a dict), or from raw bytes that still
> +    need to be deserialized.
> +
> +    :param value: Initial value, if any.
> +    :param eager: When true, attempt to serialize (or deserialize) the
> +                  initial value immediately, such that conversion exceptions
> +                  are raised during the call to the initialization method.
> +    """

Why define this class instead of using dicts? It's a very fancy way of
calling json.dumps() and json.loads().

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