python/qemu: make 'args' style arguments immutable

These arguments don't need to be mutable and aren't really used as
such. Clarify their types as immutable and adjust code to match where
necessary.

In general, It's probably best not to accept a user-defined mutable
object and store it as internal object state unless there's a strong
justification for doing so. Instead, try to use generic types as input
with empty tuples as the default, and coerce to list where necessary.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201006235817.3280413-10-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/machine.py

@@ -18,6 +18,7 @@ which provides facilities for managing the lifetime of a QEMU VM.
 #
 
 import errno
+from itertools import chain
 import logging
 import os
 import shutil
@@ -30,6 +31,8 @@ from typing import (
     Dict,
     List,
     Optional,
+    Sequence,
+    Tuple,
     Type,
 )
 
@@ -74,8 +77,12 @@ class QEMUMachine:
         # vm is guaranteed to be shut down here
     """
 
-    def __init__(self, binary, args=None, wrapper=None, name=None,
-                 test_dir="/var/tmp",
+    def __init__(self,
+                 binary: str,
+                 args: Sequence[str] = (),
+                 wrapper: Sequence[str] = (),
+                 name: Optional[str] = None,
+                 test_dir: str = "/var/tmp",
                  monitor_address: Optional[SocketAddrT] = None,
                  socket_scm_helper=None, sock_dir=None,
                  drain_console=False, console_log=None):
@@ -97,14 +104,7 @@ class QEMUMachine:
         # Direct user configuration
 
         self._binary = binary
-
-        if args is None:
-            args = []
-        # Copy mutable input: we will be modifying our copy
         self._args = list(args)
-
-        if wrapper is None:
-            wrapper = []
         self._wrapper = wrapper
 
         self._name = name or "qemu-%d" % os.getpid()
@@ -136,7 +136,7 @@ class QEMUMachine:
         self._iolog = None
         self._qmp_set = True   # Enable QMP monitor by default.
         self._qmp_connection: Optional[qmp.QEMUMonitorProtocol] = None
-        self._qemu_full_args = None
+        self._qemu_full_args: Tuple[str, ...] = ()
         self._temp_dir = None
         self._launched = False
         self._machine = None
@@ -368,7 +368,7 @@ class QEMUMachine:
             raise QEMUMachineError('VM already launched')
 
         self._iolog = None
-        self._qemu_full_args = None
+        self._qemu_full_args = ()
         try:
             self._launch()
             self._launched = True
@@ -388,8 +388,12 @@ class QEMUMachine:
         """
         devnull = open(os.path.devnull, 'rb')
         self._pre_launch()
-        self._qemu_full_args = (self._wrapper + [self._binary] +
-                                self._base_args + self._args)
+        self._qemu_full_args = tuple(
+            chain(self._wrapper,
+                  [self._binary],
+                  self._base_args,
+                  self._args)
+        )
         LOG.debug('VM launch command: %r', ' '.join(self._qemu_full_args))
         self._popen = subprocess.Popen(self._qemu_full_args,
                                        stdin=devnull,

python/qemu/qtest.py

@@ -19,7 +19,12 @@ subclass of QEMUMachine, respectively.
 
 import os
 import socket
-from typing import List, Optional, TextIO
+from typing import (
+    List,
+    Optional,
+    Sequence,
+    TextIO,
+)
 
 from .machine import QEMUMachine
 
@@ -99,8 +104,13 @@ class QEMUQtestMachine(QEMUMachine):
     A QEMU VM, with a qtest socket available.
     """
 
-    def __init__(self, binary, args=None, name=None, test_dir="/var/tmp",
-                 socket_scm_helper=None, sock_dir=None):
+    def __init__(self,
+                 binary: str,
+                 args: Sequence[str] = (),
+                 name: Optional[str] = None,
+                 test_dir: str = "/var/tmp",
+                 socket_scm_helper: Optional[str] = None,
+                 sock_dir: Optional[str] = None):
         if name is None:
             name = "qemu-%d" % os.getpid()
         if sock_dir is None:
@@ -114,8 +124,10 @@ class QEMUQtestMachine(QEMUMachine):
     @property
     def _base_args(self) -> List[str]:
         args = super()._base_args
-        args.extend(['-qtest', 'unix:path=' + self._qtest_path,
-                     '-accel', 'qtest'])
+        args.extend([
+            '-qtest', f"unix:path={self._qtest_path}",
+            '-accel', 'qtest'
+        ])
         return args
 
     def _pre_launch(self):