better test

Author: willmcgugan

src/textual/dom.py

@@ -8,6 +8,7 @@
 
 import re
 import threading
+from copy import copy
 from functools import lru_cache, partial
 from inspect import getfile
 from typing import (
@@ -334,7 +335,10 @@ def setter(value: object) -> None:
                     """Set bound data."""
                     _rich_traceback_omit = True
                     Reactive._initialize_object(self)
-                    setattr(self, variable_name, _Mutated(value))
+                    # Copy the bound value
+                    # Sharing the same instance with mutable objects can lead to confusing behavior
+                    # Wrap the value in `_Mutated` so the setter knows to invoke watchers etc
+                    setattr(self, variable_name, _Mutated(copy(value)))
 
                 return setter
 

tests/test_reactive.py

@@ -804,12 +804,25 @@ def compose(self) -> ComposeResult:
             yield TestWidget().data_bind(TestApp.messages)
 
     app = TestApp()
-    async with app.run_test() as pilot:
+    async with app.run_test():
+        test_widget = app.query_one(TestWidget)
         assert widget_messages == [[]]
+        assert test_widget.messages == []
+
+        # Previously setting a mutable object would lead to shared references
+        assert app.messages is not test_widget.messages
+
+        # Mutate app
         app.messages.append("foo")
         # Mutations aren't detected
         assert widget_messages == [[]]
+        assert app.messages == ["foo"]
+        assert test_widget.messages == []
         # Explicitly mutate app reactive
         app.mutate_reactive(TestApp.messages)
         # Mutating app, will also invoke watchers on any data binds
         assert widget_messages == [[], ["foo"]]
+        assert app.messages == ["foo"]
+        assert test_widget.messages == ["foo"]
+
+        assert app.messages is not test_widget.messages