[Fix] Resolve deprecated pipeline modules in from_pretrained

src/diffusers/pipelines/pipeline_loading_utils.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import importlib
+import importlib.util
 import os
 import re
 import warnings
@@ -1073,6 +1074,22 @@ def get_connected_passed_kwargs(prefix):
     return init_kwargs
 
 
+def _is_deprecated_pipeline_module(module_candidate: str) -> bool:
+    """Return whether ``module_candidate`` is a pipeline module relocated under
+    ``diffusers.pipelines.deprecated``.
+
+    Deprecated pipelines (e.g. Wuerstchen) are no longer attributes of ``diffusers.pipelines``, so a
+    plain ``hasattr(diffusers.pipelines, module_candidate)`` check fails for them even though the
+    module still ships with diffusers. We resolve the spec without importing the module to avoid
+    triggering its (potentially heavy) import side effects.
+    """
+    try:
+        return importlib.util.find_spec(f"diffusers.pipelines.deprecated.{module_candidate}") is not None
+    except (ImportError, ModuleNotFoundError, ValueError):
+        # ValueError covers malformed candidate names (e.g. containing path separators).
+        return False
+
+
 def _get_custom_components_and_folders(
     pretrained_model_name: str,
     config_dict: dict[str, Any],
@@ -1101,7 +1118,14 @@ def _get_custom_components_and_folders(
 
         if candidate_file in filenames:
             custom_components[component] = module_candidate
-        elif module_candidate not in LOADABLE_CLASSES and not hasattr(pipelines, module_candidate):
+        elif (
+            module_candidate not in LOADABLE_CLASSES
+            and not hasattr(pipelines, module_candidate)
+            # Pipelines moved under `diffusers.pipelines.deprecated` are no longer attributes of
+            # `diffusers.pipelines`, so `hasattr` above misses them. Check the deprecated namespace
+            # too before treating the component as a missing custom module.
+            and not _is_deprecated_pipeline_module(module_candidate)
+        ):
             raise ValueError(
                 f"{candidate_file} as defined in `model_index.json` does not exist in {pretrained_model_name} and is not a module in 'diffusers/pipelines'."
             )

tests/pipelines/test_pipeline_utils.py

@@ -18,7 +18,12 @@
     StableDiffusionPipeline,
     UNet2DConditionModel,
 )
-from diffusers.pipelines.pipeline_loading_utils import is_safetensors_compatible, variant_compatible_siblings
+from diffusers.pipelines.pipeline_loading_utils import (
+    _get_custom_components_and_folders,
+    _is_deprecated_pipeline_module,
+    is_safetensors_compatible,
+    variant_compatible_siblings,
+)
 
 from ..testing_utils import require_torch_accelerator, torch_device
 
@@ -233,6 +238,51 @@ def test_is_compatible_variant_and_non_safetensors(self):
         self.assertFalse(is_safetensors_compatible(filenames, variant="fp16"))
 
 
+class GetCustomComponentsAndFoldersTests(unittest.TestCase):
+    def test_deprecated_pipeline_module_is_recognized(self):
+        # Pipelines relocated under `diffusers.pipelines.deprecated` (e.g. Wuerstchen) are no longer
+        # attributes of `diffusers.pipelines`. They must still resolve instead of being mistaken for
+        # a missing custom module. Regression test for loading repos like `warp-ai/wuerstchen-prior`.
+        config_dict = {
+            "_class_name": "WuerstchenPriorPipeline",
+            "prior": ["wuerstchen", "WuerstchenPrior"],
+        }
+        custom_components, folder_names = _get_custom_components_and_folders(
+            "warp-ai/wuerstchen-prior", config_dict, filenames=[]
+        )
+        self.assertEqual(custom_components, {})
+        self.assertEqual(folder_names, ["prior"])
+
+    def test_missing_custom_module_still_raises(self):
+        # A component that is neither a loadable class, a known pipeline module, a deprecated module,
+        # nor an actual custom file on the Hub must still raise.
+        config_dict = {
+            "_class_name": "FooPipeline",
+            "foo": ["totally_made_up_module", "FooModel"],
+        }
+        with self.assertRaises(ValueError):
+            _get_custom_components_and_folders("some/repo", config_dict, filenames=[])
+
+    def test_custom_component_file_is_detected(self):
+        # When the custom module file is actually present on the Hub it is recorded as a custom component.
+        config_dict = {
+            "_class_name": "FooPipeline",
+            "foo": ["my_pipeline", "FooModel"],
+        }
+        custom_components, folder_names = _get_custom_components_and_folders(
+            "some/repo", config_dict, filenames=["foo/my_pipeline.py"]
+        )
+        self.assertEqual(custom_components, {"foo": "my_pipeline"})
+
+    def test_is_deprecated_pipeline_module(self):
+        self.assertTrue(_is_deprecated_pipeline_module("wuerstchen"))
+        self.assertFalse(_is_deprecated_pipeline_module("totally_made_up_module"))
+        # A non-deprecated (current) pipeline module is not under the deprecated namespace.
+        self.assertFalse(_is_deprecated_pipeline_module("stable_diffusion"))
+        # Malformed candidate names must not raise.
+        self.assertFalse(_is_deprecated_pipeline_module("weird/name"))
+
+
 class VariantCompatibleSiblingsTest(unittest.TestCase):
     def test_only_non_variants_downloaded(self):
         ignore_patterns = ["*.bin"]