refactor: make default select use overrides, raise if overridden bloc…

…k not found

xblock/plugin.py

@@ -28,7 +28,7 @@ def __init__(self, all_entry_points):
         super().__init__(msg)
 
 
-def default_select(identifier, all_entry_points):  # pylint: disable=inconsistent-return-statements
+def _default_select_no_override(identifier, all_entry_points):  # pylint: disable=inconsistent-return-statements
     """
     Raise an exception when we have ambiguous entry points.
     """
@@ -41,7 +41,7 @@ def default_select(identifier, all_entry_points):  # pylint: disable=inconsisten
         raise AmbiguousPluginError(all_entry_points)
 
 
-def select_with_overrides(identifier, all_entry_points):
+def default_select(identifier, all_entry_points):
     """
     Honors the ability for an XBlock to override the default entry point.
     Raise an exception when we have ambiguous entry points.
@@ -57,14 +57,15 @@ def select_with_overrides(identifier, all_entry_points):
         else:
             block_entry_points.append(block_entry_point)
 
-    # Overrides get priority
+    # Get the default entry point
+    default_plugin = _default_select_no_override(identifier, block_entry_points)
+
+    # If we have an unambiguous override, that gets priority. Otherwise, return default.
     if len(overrides) == 1:
         return overrides[0]
     elif len(overrides) > 1:
         raise AmbiguousPluginError(all_entry_points)
-
-    # ... then default behavior
-    return default_select(identifier, block_entry_points)
+    return default_plugin
 
 
 class Plugin:
@@ -124,7 +125,7 @@ def select(identifier, all_entry_points):
         if key not in PLUGIN_CACHE:
 
             if select is None:
-                select = select_with_overrides
+                select = default_select
 
             all_entry_points = [
                 *importlib.metadata.entry_points(group=f'{cls.entry_point}.overrides', name=identifier),

xblock/test/test_plugin.py

@@ -57,14 +57,8 @@ def boom(identifier, entry_points):
 
 
 @XBlock.register_temp_plugin(OverriddenBlock, "overridden_block", group='xblock.v1.overrides')
-@XBlock.register_temp_plugin(AmbiguousBlock1, "overridden_block")
-@XBlock.register_temp_plugin(AmbiguousBlock2, "overridden_block")
-@XBlock.register_temp_plugin(UnambiguousBlock, "good_block")
+@XBlock.register_temp_plugin(UnambiguousBlock, "overridden_block")
 def test_plugin_override():
-    # We can load ok blocks even if there are bad blocks.
-    cls = XBlock.load_class("good_block")
-    assert cls is UnambiguousBlock
-
     # Trying to load a block that is overridden returns the correct override
     override = XBlock.load_class("overridden_block")
     assert override is OverriddenBlock