python/qemu: Adjust traceback typing

mypy considers it incorrect to use `bool` to statically return false,
because it will assume that it could conceivably return True, and gives
different analysis in that case. Use a None return to achieve the same
effect, but make mypy happy.

Note: Pylint considers function signatures as code that might trip the
duplicate-code checker. I'd rather not disable this as it does not
trigger often in practice, so I'm disabling it as a one-off and filed a
change request; see pylint-dev/pylint#3619

Signed-off-by: John Snow <jsnow@redhat.com>
Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200514055403.18902-14-jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

python/qemu/machine.py

@@ -24,6 +24,8 @@
 import shutil
 import socket
 import tempfile
+from typing import Optional, Type
+from types import TracebackType
 
 from . import qmp
 
@@ -124,9 +126,11 @@ def __init__(self, binary, args=None, wrapper=None, name=None,
     def __enter__(self):
         return self
 
-    def __exit__(self, exc_type, exc_val, exc_tb):
+    def __exit__(self,
+                 exc_type: Optional[Type[BaseException]],
+                 exc_val: Optional[BaseException],
+                 exc_tb: Optional[TracebackType]) -> None:
         self.shutdown()
-        return False
 
     def add_monitor_null(self):
         """

python/qemu/qmp.py

@@ -14,7 +14,9 @@
 from typing import (
     Optional,
     TextIO,
+    Type,
 )
+from types import TracebackType
 
 
 class QMPError(Exception):
@@ -146,10 +148,14 @@ def __enter__(self):
         # Implement context manager enter function.
         return self
 
-    def __exit__(self, exc_type, exc_value, exc_traceback):
+    def __exit__(self,
+                 # pylint: disable=duplicate-code
+                 # see https://github.com/PyCQA/pylint/issues/3619
+                 exc_type: Optional[Type[BaseException]],
+                 exc_val: Optional[BaseException],
+                 exc_tb: Optional[TracebackType]) -> None:
         # Implement context manager exit function.
         self.close()
-        return False
 
     def connect(self, negotiate=True):
         """