cache: Simplify invalidation logic to match existing behaviour

confuse/cache.py

@@ -7,9 +7,8 @@
 class CachedHandle(object):
     """Handle for a cached value computed by applying a template on the view.
     """
-    # some sentinel objects
     _INVALID = object()
-    _MISSING = object()
+    """Sentinel object to denote that the cached value if out-of-date."""
 
     def __init__(self, view: ConfigView, template=templates.REQUIRED) -> None:
         self.value = self._INVALID
@@ -22,12 +21,8 @@ def get(self):
         Will re-compute the value using `view.get(template)` if it has been
         invalidated.
 
-        May raise a `NotFoundError` if the underlying view has been
-        invalidated.
+        May raise a `NotFoundError` if the underlying view is missing.
         """
-        if self.value is self._MISSING:
-            # will raise a NotFoundError if no default value was provided
-            self.value = templates.as_template(self.template).get_default_value()
         if self.value is self._INVALID:
             self.value = self.view.get(self.template)
         return self.value
@@ -37,11 +32,6 @@ def _invalidate(self):
         """
         self.value = self._INVALID
 
-    def _set_view_missing(self):
-        """Invalidate the handle, will raise `NotFoundError` on `get()`.
-        """
-        self.value = self._MISSING
-
 
 class CachedViewMixin:
     def __init__(self, *args, **kwargs):
@@ -62,9 +52,7 @@ def __getitem__(self, key) -> "CachedConfigView":
     def __setitem__(self, key, value):
         subview: CachedConfigView = self[key]
         # invalidate the existing handles up and down the view tree
-        for handle in subview.handles:
-            handle._invalidate()
-        subview._invalidate_descendants(value)
+        subview._invalidate_descendants()
         self._invalidate_ancestors()
 
         return super().__setitem__(key, value)
@@ -82,24 +70,13 @@ def _invalidate_ancestors(self):
                 break
             parent = parent.parent
 
-    def _invalidate_descendants(self, new_val):
-        """Invalidate the handles for (sub)keys that were updated and
-        set_view_missing for keys that are absent in new_val.
+    def _invalidate_descendants(self):
+        """Invalidate the handles for (sub)keys that were updated.
         """
+        for handle in self.handles:
+            handle._invalidate()
         for subview in self.subviews.values():
-            try:
-                subval = new_val[subview.key]
-            except (KeyError, IndexError, TypeError):
-                # the old key doesn't exist in the new value anymore- 
-                # set view as missing for the handles.
-                for handle in subview.handles:
-                    handle._set_view_missing()
-                subval = None
-            else:
-                # old key is present, possibly with a new value- invalidate.
-                for handle in subview.handles:
-                    handle._invalidate()
-            subview._invalidate_descendants(subval)
+            subview._invalidate_descendants()
 
     def get_handle(self, template=templates.REQUIRED):
         """Retreive a `CachedHandle` for the current view and template.

test/test_cache.py

@@ -2,7 +2,6 @@
 
 import confuse
 from confuse.cache import CachedConfigView, CachedHandle, CachedRootView
-from confuse.exceptions import NotFoundError
 from confuse.templates import Sequence
 
 
@@ -35,19 +34,17 @@ def test_missing(self):
         handle: CachedHandle = view.get_handle(Sequence(int))
 
         self.config['x'] = {'p': [4, 5]}
-        # new dict doesn't have a 'y' key
-        with self.assertRaises(NotFoundError):
-            handle.get()
+        # new dict doesn't have a 'y' key, but according to the view-theory,
+        # it will get the value from the older view that has been shadowed.
+        self.assertEqual(handle.get(), [1, 2])
 
     def test_missing2(self):
         view: CachedConfigView = self.config['x']['w']
         handle = view.get_handle(str)
         self.assertEqual(handle.get(), 'z')
 
         self.config['x'] = {'y': [4, 5]}
-        # new dict doesn't have a 'w' key
-        with self.assertRaises(NotFoundError):
-            handle.get()
+        self.assertEqual(handle.get(), 'z')
 
     def test_list_update(self):
         view: CachedConfigView = self.config['a'][1]