change how fallback_value is used

mesa/experimental/signals/observable_collections.py

@@ -51,7 +51,7 @@ def __set__(self, instance: "HasObservables", value: Iterable):
 
         """
         instance.notify(
-            self.public_name, self.__get__(instance, None), value, "on_change"
+            self.public_name, getattr(instance, self.private_name, self.fallback_value), value, "on_change"
         )
         setattr(
             instance,

mesa/experimental/signals/signal.py

@@ -54,6 +54,9 @@ def __get__(self, instance, owner):
 
 class Observable:
     """Base Observable class."""
+    # fixme, we might want to have a base observable
+    #  and move some of this into this class and use super to allways use it
+    #  for example the on_change notify can go there
 
     def __init__(self):
         """Initialize an Observable."""
@@ -67,9 +70,7 @@ def __init__(self):
         self.fallback_value = None  # fixme, should this be user specifiable
 
     def __get__(self, instance, owner):  # noqa D103
-        # fixme how do we want to handle the fallback value
-        # and when should it raise an attribute error?
-        return getattr(instance, self.private_name, self.fallback_value)
+        return getattr(instance, self.private_name)
 
     def __set_name__(self, owner, name):  # noqa D103
         self.public_name = name
@@ -78,7 +79,7 @@ def __set_name__(self, owner, name):  # noqa D103
 
     def __set__(self, instance: "HasObservables", value):  # noqa D103
         instance.notify(
-            self.public_name, self.__get__(instance, None), value, "on_change"
+            self.public_name, getattr(instance, self.private_name, self.fallback_value), value, "on_change"
         )
         setattr(instance, self.private_name, value)
 
@@ -194,6 +195,7 @@ def unobserve(self, name: str | All, signal_type: str | All):
             if not isinstance(name, All)
             else self.observables.keys()
         )
+
         if isinstance(signal_type, All):
             signal_types = self.observables[name].signal_types
         else:
@@ -222,7 +224,7 @@ def unobserve_all(self, name: str | All):
                 # ignore when unsubscribing to  Observables that have no subscription
         else:
             self.subscribers = defaultdict(
-                functools.partial(defaultdict, weakref.WeakValueDictionary)
+                functools.partial(defaultdict, weakref.WeakSet)
             )
 
     def notify(self, observable: str, old_value: Any, new_value: Any, signal_type: str):
@@ -237,7 +239,9 @@ def notify(self, observable: str, old_value: Any, new_value: Any, signal_type: s
         """
         # fixme: currently strongly tied to just on_change signals
         #  this needs to be refined for e.g. list and dicts in due course
-        #  idea is to just mimic how traitlets handles this
+        #  idea is to just mimic how traitlets handles this.
+        #  Traitlets uses a Bunch helper class which turns a dict into something with
+        #  attribute access. This will be richer than the current Signal named tuple
         signal = Signal(self, observable, old_value, new_value, signal_type)
 
         observers = self.subscribers[observable][signal_type]