python: backport 'EventListener: add __repr__ method'

When the object is not stateful, this repr method prints what you'd
expect. In cases where there are pending events, the output is augmented
to illustrate that.

The object itself has no idea if it's "active" or not, so it cannot
convey that information.

Signed-off-by: John Snow <jsnow@redhat.com>
cherry picked from commit python-qemu-qmp@8a6f2e136dae395fec8aa5fd77487cfe12d9e05e
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

python/qemu/qmp/events.py

@@ -497,6 +497,21 @@ class EventListener:
         #: Optional, secondary event filter.
         self.event_filter: Optional[EventFilter] = event_filter
 
+    def __repr__(self) -> str:
+        args: List[str] = []
+        if self.names:
+            args.append(f"names={self.names!r}")
+        if self.event_filter:
+            args.append(f"event_filter={self.event_filter!r}")
+
+        if self._queue.qsize():
+            state = f"<pending={self._queue.qsize()}>"
+        else:
+            state = ''
+
+        argstr = ", ".join(args)
+        return f"{type(self).__name__}{state}({argstr})"
+
     @property
     def history(self) -> Tuple[Message, ...]:
         """