Do not fail to disable a component with an unimportable path

`get_component_priority_dict_with_base()` normalizes every key of a
component priority dictionary by calling `load_object()` on it, including
keys whose value is `None`, i.e. components the user is disabling.

`normalize_key()` only guarded against `(NameError, TypeError, ValueError)`,
the three exceptions `load_object()` raises itself, but not against the
`ImportError` raised by the `import_module()` call it makes. A stale entry
disabling a component whose module no longer exists, e.g.
`{"scrapy.webservice.WebService": None}`, therefore aborted settings
resolution with `ModuleNotFoundError` instead of being ignored.

Add `ImportError` to the guard so that an unimportable key falls back to its
raw string form. Since its value is `None`, it is then dropped by the
existing `if v is not None` filter, and disabling by import path keeps
working. Enabled components with an unimportable path are unaffected:
`MiddlewareManager.from_crawler()` calls `load_object()` on them again and
still fails there.

Fixes #7820

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/news.rst

@@ -25,6 +25,16 @@ Backward-incompatible changes
 
     (:issue:`6585`, :issue:`7731`)
 
+Bug fixes
+~~~~~~~~~
+
+-   Disabling a component whose import path cannot be imported, e.g. a
+    component removed from a later Scrapy version, no longer raises
+    :exc:`ModuleNotFoundError` while resolving :ref:`component priority
+    dictionaries <component-priority-dictionaries>`. This bug was introduced in
+    Scrapy 2.15.0.
+    (:issue:`7820`)
+
 .. _release-2.17.0:
 
 Scrapy 2.17.0 (2026-07-07)

scrapy/settings/__init__.py

@@ -366,7 +366,7 @@ class BaseSettings(MutableMapping[str, Any]):
         def normalize_key(key: Any) -> Any:
             try:
                 loaded_key = load_object(key)
-            except (NameError, TypeError, ValueError):
+            except (ImportError, NameError, TypeError, ValueError):
                 loaded_key = key
             else:
                 import_path = global_object_name(loaded_key)

tests/test_settings/__init__.py

@@ -428,6 +428,9 @@ class TestBaseSettings:
             pytest.param(1, TypeError, id="type-error"),
             pytest.param("foo", ValueError, id="value-error"),
             pytest.param("csv.gz", NameError, id="name-error"),
+            pytest.param(
+                "nonexistent.module.Component", ImportError, id="import-error"
+            ),
         ],
     )
     def test_get_component_priority_dict_with_base_handles_load_object_exceptions(
@@ -446,6 +449,20 @@ class TestBaseSettings:
         assert isinstance(value, BaseSettings)
         assert dict(value) == {key: 1}
 
+    def test_get_component_priority_dict_with_base_disable_unimportable_key(self):
+        """Disabling a component whose module cannot be imported, e.g. one
+        removed from a later Scrapy version, must not break settings
+        resolution."""
+        settings = BaseSettings(
+            {
+                "FOO_BASE": BaseSettings({"csv.excel": 1}),
+                "FOO": BaseSettings({"nonexistent.module.Component": None}),
+            }
+        )
+        value = settings.get_component_priority_dict_with_base("FOO")
+
+        assert dict(value) == {"csv.excel": 1}
+
     def test_get_component_priority_dict_with_base_override_none_by_type(self):
         settings = BaseSettings()
         setting_names = set()