[PATCH v4 12/16] python/machine.py: Add _qmp access shim

[John Snow <jsnow@redhat.com>]

Like many other Optional[] types, it's not always a given that this
object will be set. Wrap it in a type-shim that raises a meaningful
error and will always return a concrete type.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 python/qemu/machine.py | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/python/qemu/machine.py b/python/qemu/machine.py
index 9bb5d26407..cb3db1f66a 100644
--- a/python/qemu/machine.py
+++ b/python/qemu/machine.py
@@ -118,7 +118,7 @@ def __init__(self, binary, args=None, wrapper=None, name=None,
         self._events = []
         self._iolog = None
         self._qmp_set = True   # Enable QMP monitor by default.
-        self._qmp = None
+        self._qmp_connection: Optional[qmp.QEMUMonitorProtocol] = None
         self._qemu_full_args = None
         self._temp_dir = None
         self._launched = False
@@ -285,14 +285,14 @@ def _pre_launch(self):
             if self._remove_monitor_sockfile:
                 assert isinstance(self._monitor_address, str)
                 self._remove_files.append(self._monitor_address)
-            self._qmp = qmp.QEMUMonitorProtocol(
+            self._qmp_connection = qmp.QEMUMonitorProtocol(
                 self._monitor_address,
                 server=True,
                 nickname=self._name
             )
 
     def _post_launch(self):
-        if self._qmp:
+        if self._qmp_connection:
             self._qmp.accept()
 
     def _post_shutdown(self) -> None:
@@ -370,7 +370,7 @@ def wait(self):
         Wait for the VM to power off
         """
         self._popen.wait()
-        if self._qmp:
+        if self._qmp_connection:
             self._qmp.close()
         self._post_shutdown()
 
@@ -459,11 +459,13 @@ def set_qmp_monitor(self, enabled=True):
                         line. Default is True.
         @note: call this function before launch().
         """
-        if enabled:
-            self._qmp_set = True
-        else:
-            self._qmp_set = False
-            self._qmp = None
+        self._qmp_set = enabled
+
+    @property
+    def _qmp(self) -> qmp.QEMUMonitorProtocol:
+        if self._qmp_connection is None:
+            raise QEMUMachineError("Attempt to access QMP with no connection")
+        return self._qmp_connection
 
     @classmethod
     def _qmp_args(cls, _conv_keys: bool = True, **args: Any) -> Dict[str, Any]:
-- 
2.21.3