Make variables bind weakly

src/pybind/values.py

@@ -71,7 +71,7 @@ def value(self, new_value: _S) -> None:
         raise NotImplementedError
 
     @abstractmethod
-    def bind_to(self, value: Value[_S], already_bound_ok: bool = False) -> None:
+    def bind_to(self, value: Value[_S], already_bound_ok: bool = False, bind_weakly: bool = True) -> None:
         raise NotImplementedError
 
     @abstractmethod
@@ -117,7 +117,7 @@ def weak_observe(self, observer: Observer | ValueObserver[_S]) -> None:
     def unobserve(self, observer: Observer | ValueObserver[_S]) -> None:
         self._on_change.unobserve(observer)
 
-    def bind_to(self, value: Value[_S], already_bound_ok: bool = False) -> None:
+    def bind_to(self, value: Value[_S], already_bound_ok: bool = False, bind_weakly: bool = True) -> None:
         if value is self:
             raise RecursionError("Cannot bind a Variable to itself.")
         if value.is_dependent_on(self):
@@ -128,7 +128,10 @@ def bind_to(self, value: Value[_S], already_bound_ok: bool = False) -> None:
             if self._bound_to is value:
                 return
             self.unbind()
-        value.observe(self._receive_bound_value)
+        if bind_weakly:
+            value.weak_observe(self._receive_bound_value)
+        else:
+            value.observe(self._receive_bound_value)
         self._bound_to = value
         self._bound_to_set = frozenset([value])
         self._set_value_bypass_bound_check(value.value)

tests/test_simple_variable.py

@@ -1,3 +1,5 @@
+import gc
+
 import pytest
 from pybind.values import SimpleVariable, Constant
 from conftest import NoParametersObserver, OneParameterObserver
@@ -313,13 +315,52 @@ def test_simple_variable_deep_derived_from_diamond_pattern():
     assert variable_top in dependencies
 
 
-def test_constant_derived_from_empty():
-    constant = Constant("test")
+def test_bind_weak_reference_clears():
+    root_var = SimpleVariable("root0")
+    dependent_var = SimpleVariable("")
+    dependent_var.bind_to(root_var, bind_weakly=True)
+
+    values = []
+    dependent_var.observe(lambda value: values.append(value))
+
+    root_var.value = "root1"
+    dependent_var = None
+    gc.collect()
+    root_var.value = "root2"
+
+    assert values == ["root1"]
+
+
+def test_bind_strong_reference_stays():
+    root_var = SimpleVariable("root0")
+    dependent_var = SimpleVariable("")
+    dependent_var.bind_to(root_var, bind_weakly=False)
+
+    values = []
+    dependent_var.observe(lambda value: values.append(value))
+
+    root_var.value = "root1"
+    dependent_var = None
+    gc.collect()
+    root_var.value = "root2"
+
+    assert values == ["root1", "root2"]
+
+
+def test_daisy_chain_variables_weak_reference_stays():
+    root_var = SimpleVariable("root0")
+    middle_var = SimpleVariable("")
+    dependent_var = SimpleVariable("")
 
-    assert constant.derived_from() == frozenset()
+    middle_var.bind_to(root_var, bind_weakly=True)
+    dependent_var.bind_to(middle_var, bind_weakly=False)
 
+    values = []
+    dependent_var.observe(lambda value: values.append(value))
 
-def test_constant_deep_derived_from_empty():
-    constant = Constant("test")
+    root_var.value = "root1"
+    middle_var = None
+    gc.collect()
+    root_var.value = "root2"
 
-    assert list(constant.deep_derived_from) == []
+    assert values == ["root1", "root2"]