Create API for Tracker; add comments

netbox/utilities/counter.py

@@ -14,9 +14,9 @@ def post_save_receiver_counter(counter_instance, sender, instance, created, **kw
 
     # not created so check if field has changed
     field_name = f"{counter_instance.foreign_key_field.name}_id"
-    if field_name in instance.tracker.changed:
+    if field_name in instance.tracker:
         new_value = getattr(instance, field_name, None)
-        old_value = instance.tracker.changed[field_name]
+        old_value = instance.tracker.get(field_name)
         if (new_value is not None) and (new_value != old_value):
             counter_instance.adjust_count(new_value, 1)
             counter_instance.adjust_count(old_value, -1)

netbox/utilities/mixins.py

@@ -4,46 +4,75 @@ from netbox.registry import registry
 
 
 class Tracker:
-    def __init__(self, instance):
-        self.instance = instance
-        self.changed = {}
+    """
+    An ephemeral instance employed to record which tracked fields on an instance have been modified.
+    """
+    def __init__(self):
+        self._changed_fields = {}
+
+    def __contains__(self, item):
+        return item in self._changed_fields
+
+    def set(self, name, value):
+        """
+        Mark an attribute as having been changed and record its original value.
+        """
+        self._changed_fields[name] = value
+
+    def get(self, name):
+        """
+        Return the original value of a changed field. Raises KeyError if name is not found.
+        """
+        return self._changed_fields[name]
+
+    def clear(self, *names):
+        """
+        Clear any fields that were recorded as having been changed.
+        """
+        for name in names:
+            self._changed_fields.pop(name, None)
+        else:
+            self._changed_fields = {}
 
 
 class TrackingModelMixin:
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
+
+        # Mark the instance as initialized, to enable our custom __setattr__()
         self._initialized = True
 
     @property
     def tracker(self):
+        """
+        Return the Tracker instance for this instance, first creating it if necessary.
+        """
         if not hasattr(self._state, "_tracker"):
-            self._state._tracker = Tracker(self)
+            self._state._tracker = Tracker()
         return self._state._tracker
 
     def save(self, *args, **kwargs):
         super().save(*args, **kwargs)
 
-        if self.tracker.changed:
-            if update_fields := kwargs.get('update_fields', None):
-                for field in update_fields:
-                    self.tracker.changed.pop(field, None)
-            else:
-                self.tracker.changed = {}
+        # Clear any tracked fields now that changes have been saved
+        update_fields = kwargs.get('update_fields', [])
+        self.tracker.clear(*update_fields)
 
     def __setattr__(self, name, value):
         if hasattr(self, "_initialized"):
-            change_tracking_fields = registry['counter_fields'][self.__class__]
-            if name in change_tracking_fields:
-                if name not in self.tracker.changed:
+            # Record any changes to a tracked field
+            if name in registry['counter_fields'][self.__class__]:
+                if name not in self.tracker:
+                    # The attribute has been created or changed
                     if name in self.__dict__:
                         old_value = getattr(self, name)
                         if value != old_value:
-                            self.tracker.changed[name] = old_value
+                            self.tracker.set(name, old_value)
                     else:
-                        self.tracker.changed[name] = DeferredAttribute
-                else:
-                    if value == self.tracker.changed[name]:
-                        self.tracker.changed.pop(name)
+                        self.tracker.set(name, DeferredAttribute)
+                elif value == self.tracker.get(name):
+                    # A previously changed attribute has been restored
+                    self.tracker.clear(name)
 
         super().__setattr__(name, value)