python/aqmp: add error classes

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20210915162955.333025-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/aqmp/__init__.py

@@ -21,7 +21,11 @@ managing QMP events.
 # This work is licensed under the terms of the GNU GPL, version 2.  See
 # the COPYING file in the top-level directory.
 
+from .error import AQMPError
+
 
 # The order of these fields impact the Sphinx documentation order.
 __all__ = (
+    # Exceptions
+    'AQMPError',
 )

python/qemu/aqmp/error.py

@@ -0,0 +1,50 @@
+"""
+AQMP Error Classes
+
+This package seeks to provide semantic error classes that are intended
+to be used directly by clients when they would like to handle particular
+semantic failures (e.g. "failed to connect") without needing to know the
+enumeration of possible reasons for that failure.
+
+AQMPError serves as the ancestor for all exceptions raised by this
+package, and is suitable for use in handling semantic errors from this
+library. In most cases, individual public methods will attempt to catch
+and re-encapsulate various exceptions to provide a semantic
+error-handling interface.
+
+.. admonition:: AQMP Exception Hierarchy Reference
+
+ |   `Exception`
+ |    +-- `AQMPError`
+ |         +-- `ConnectError`
+ |         +-- `StateError`
+ |         +-- `ExecInterruptedError`
+ |         +-- `ExecuteError`
+ |         +-- `ListenerError`
+ |         +-- `ProtocolError`
+ |              +-- `DeserializationError`
+ |              +-- `UnexpectedTypeError`
+ |              +-- `ServerParseError`
+ |              +-- `BadReplyError`
+ |              +-- `GreetingError`
+ |              +-- `NegotiationError`
+"""
+
+
+class AQMPError(Exception):
+    """Abstract error class for all errors originating from this package."""
+
+
+class ProtocolError(AQMPError):
+    """
+    Abstract error class for protocol failures.
+
+    Semantically, these errors are generally the fault of either the
+    protocol server or as a result of a bug in this library.
+
+    :param error_message: Human-readable string describing the error.
+    """
+    def __init__(self, error_message: str):
+        super().__init__(error_message)
+        #: Human-readable error message, without any prefix.
+        self.error_message: str = error_message