Make derived values bind weakly

.flake8

@@ -9,4 +9,4 @@ exclude =
 
 # Allow assigning lambdas in tests
 per-file-ignores =
-    tests/*:E731
+    tests/*:E731,F841

src/spellbind/float_values.py

@@ -133,20 +133,22 @@ def _get_float(value: float | Value[int] | Value[float]) -> float:
 class CombinedFloatValues(DerivedValueBase[_U], Generic[_U], ABC):
     def __init__(self, *values: float | Value[int] | Value[float]):
         super().__init__(*[v for v in values if isinstance(v, Value)])
-        self.gotten_values = [_get_float(v) for v in values]
+        self._gotten_values = [_get_float(v) for v in values]
+        self._callbacks: list[Callable] = []
         for i, v in enumerate(values):
             if isinstance(v, Value):
-                v.observe(self._create_on_n_changed(i))
+                v.weak_observe(self._create_on_n_changed(i))
         self._value = self._calculate_value()
 
     def _create_on_n_changed(self, index: int) -> Callable[[float], None]:
         def on_change(new_value: float) -> None:
-            self.gotten_values[index] = new_value
+            self._gotten_values[index] = new_value
             self._on_result_change(self._calculate_value())
+        self._callbacks.append(on_change)  # keep strong reference to callback so it won't be garbage collected
         return on_change
 
     def _calculate_value(self) -> _U:
-        return self.transform(self.gotten_values)
+        return self.transform(self._gotten_values)
 
     def _on_result_change(self, new_value: _U) -> None:
         if new_value != self._value:

src/spellbind/values.py

@@ -215,7 +215,7 @@ class DerivedValue(DerivedValueBase[_T], Generic[_S, _T], ABC):
     def __init__(self, of: Value[_S]):
         super().__init__(of)
         self._value = self.transform(of.value)
-        of.observe(self._on_source_change)
+        of.weak_observe(self._on_source_change)
 
     @abstractmethod
     def transform(self, value: _S) -> _T:
@@ -257,9 +257,9 @@ def __init__(self, left: Value[_S] | _S, right: Value[_T] | _T):
         self._left_getter = _create_value_getter(left)
         self._right_getter = _create_value_getter(right)
         if isinstance(left, Value):
-            left.observe(self._on_left_change)
+            left.weak_observe(self._on_left_change)
         if isinstance(right, Value):
-            right.observe(self._on_right_change)
+            right.weak_observe(self._on_right_change)
         self._value = self.transform(self._left_getter(), self._right_getter())
 
     def _on_left_change(self, new_left_value: _S) -> None:
@@ -295,15 +295,17 @@ class CombinedMixedValues(DerivedValueBase[_T], Generic[_S, _T], ABC):
     def __init__(self, *sources: Value[_S] | _S):
         super().__init__(*[v for v in sources if isinstance(v, Value)])
         self.gotten_values = [_get_value(v) for v in sources]
+        self._callbacks: list[Callable] = []
         for i, v in enumerate(sources):
             if isinstance(v, Value):
-                v.observe(self._create_on_n_changed(i))
+                v.weak_observe(self._create_on_n_changed(i))
         self._value = self._calculate_value()
 
     def _create_on_n_changed(self, index: int) -> Callable[[_S], None]:
         def on_change(new_value: _S) -> None:
             self.gotten_values[index] = new_value
             self._on_result_change(self._calculate_value())
+        self._callbacks.append(on_change)  # keep strong reference to callback so it won't be garbage collected
         return on_change
 
     def _calculate_value(self) -> _T:

tests/test_values/test_float_values/test_float_values.py

@@ -1,4 +1,6 @@
-from spellbind.float_values import FloatConstant, MaxFloatValues, MinFloatValues
+import gc
+
+from spellbind.float_values import FloatConstant, MaxFloatValues, MinFloatValues, FloatVariable
 from spellbind.values import SimpleVariable
 
 
@@ -49,3 +51,28 @@ def test_min_float_values_with_literals():
 
     a.value = 5.1
     assert min_val.value == 5.1
+
+
+def test_add_float_values_keeps_reference():
+    v0 = FloatVariable(1.5)
+    v1 = FloatVariable(2.5)
+    v2 = v0 + v1
+    assert len(v0._on_change._subscriptions) == 1
+    gc.collect()
+
+    v0.value = 3.5
+    assert len(v0._on_change._subscriptions) == 1
+
+
+def test_add_int_values_garbage_collected():
+    v0 = FloatVariable(1.5)
+    v1 = FloatVariable(2.5)
+    v2 = v0 + v1
+    assert len(v0._on_change._subscriptions) == 1
+    assert len(v1._on_change._subscriptions) == 1
+    v2 = None
+    gc.collect()
+    v0.value = 3.5  # trigger removal of weak references
+    v1.value = 4.5  # trigger removal of weak references
+    assert len(v0._on_change._subscriptions) == 0
+    assert len(v1._on_change._subscriptions) == 0

tests/test_values/test_int_values/test_add_int_values.py

@@ -1,3 +1,5 @@
+import gc
+
 from spellbind.float_values import FloatVariable
 from spellbind.int_values import IntVariable
 
@@ -56,3 +58,29 @@ def test_add_float_plus_int_value():
 
     v1.value = 4
     assert v2.value == 7.5
+
+
+def test_add_int_values_keeps_reference():
+    v0 = IntVariable(1)
+    v1 = IntVariable(2)
+    v2 = v0 + v1
+    assert len(v0._on_change._subscriptions) == 1
+    gc.collect()
+
+    v0.value = 3
+    v1.value = 4
+    assert len(v0._on_change._subscriptions) == 1
+
+
+def test_add_int_values_garbage_collected():
+    v0 = IntVariable(1)
+    v1 = IntVariable(2)
+    v2 = v0 + v1
+    assert len(v0._on_change._subscriptions) == 1
+    assert len(v1._on_change._subscriptions) == 1
+    v2 = None
+    gc.collect()
+    v0.value = 3  # trigger removal of weak references
+    v1.value = 4  # trigger removal of weak references
+    assert len(v0._on_change._subscriptions) == 0
+    assert len(v1._on_change._subscriptions) == 0