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

From: John Snow
Subject: Re: [PATCH RFC 4/7] message: add QMP Message type
Date: Wed, 14 Apr 2021 13:39:07 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 4/13/21 4:07 PM, Stefan Hajnoczi wrote:
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().

Mostly just to associate the de/serialization methods of the unit-message with that data type, and it's nice for strict typing.

It does repeat a lot of boilerplate to just re-implement the dict-quacking; but I think I might actually be able to get around that by inheriting from MutableMapping to get all of that boilerplate "for free".

I'll see. I'll put it high on the list for the chopping block.


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