fix #2979: use dictionary for mutable tracking

Using a dictionary instead of a list keeps only one working copy,
allowing consistency in between calls of update_mutables()

qutebrowser/config/config.py

@@ -368,7 +368,7 @@ class Config(QObject):
 
     Attributes:
         _values: A dict mapping setting names to their values.
-        _mutables: A list of mutable objects to be checked for changes.
+        _mutables: A dictionary of mutable objects to be checked for changes.
         _yaml: A YamlConfig object or None.
 
     Signals:
@@ -430,19 +430,23 @@ class Config(QObject):
         If mutable=True is set, watch the returned object for mutations.
         """
         opt = self.get_opt(name)
-        # We always return a copy of the value stored internally, so the
+        obj = None
+        # If we allow mutation, there is a chance that prior mutations already
+        # entered the mutable dictionary and thus further copies are unneeded
+        # until update_mutables() is called
+        if name in self._mutables and mutable:
+            obj = self._mutables[name][1]
+        # Otherwise, we return a copy of the value stored internally, so the
         # internal value can never be changed by mutating the object returned.
-        obj = copy.deepcopy(self._values.get(name, opt.default))
-        # Then we watch the returned object for changes.
-        if isinstance(obj, (dict, list)):
-            if mutable:
-                if name not in self._mutables:
-                    self._mutables[name] = (copy.deepcopy(obj), obj)
-                else:
-                    obj = self._mutables[name][1]
         else:
-            # Shouldn't be mutable (and thus hashable)
-            assert obj.__hash__ is not None, obj
+            obj = copy.deepcopy(self._values.get(name, opt.default))
+            # Then we watch the returned object for changes.
+            if isinstance(obj, (dict, list)):
+                if mutable:
+                    self._mutables[name] = (copy.deepcopy(obj), obj)
+            else:
+                # Shouldn't be mutable (and thus hashable)
+                assert obj.__hash__ is not None, obj
         return obj
 
     def get_str(self, name):

tests/unit/config/test_config.py

@@ -706,7 +706,7 @@ class TestConfig:
                         assert obj == new
 
         if mutable:
-            assert conf._mutables == ( old, new)
+            assert conf._mutables[option] == (old, new)
 
         if mutable and mutated:
             # Now let's update