Fix get_flow() with reinstantiate=True for flows without extensions

- Modify get_flow() to gracefully handle flows without registered extensions when reinstantiate=True is passed - Log a warning instead of raising ValueError when no extension is available - Update docstring to document the new behavior - Add tests for OpenML-native flows and flows without extensions - Fixes issue #1626 Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>
openml · ssudhiravinesh · Mar 7, 2026 · Mar 7, 2026 · 1c04aedb91af051c42e46454d71adb544df0c05a
commit 1c04aedb91af051c42e46454d71adb544df0c05a
diff --git a/openml/flows/functions.py b/openml/flows/functions.py
@@ -1,6 +1,7 @@
 # License: BSD 3-Clause
 from __future__ import annotations
 
+import logging
 import os
 import re
 from collections import OrderedDict
@@ -83,9 +84,11 @@ def get_flow(flow_id: int, reinstantiate: bool = False, strict_version: bool = T
     flow_id : int
         The OpenML flow id.
     reinstantiate : bool, optional (default=False)
-        If True, convert the flow description into a concrete model instance
-        using the flow's extension (e.g., sklearn). If conversion fails and
-        ``strict_version`` is True, an exception will be raised.
+        If True, attempt to convert the flow description into a concrete model
+        instance using the flow's extension (e.g., sklearn). If no extension is
+        registered for the flow type (e.g., for OpenML-native evaluation functions
+        or flows from uninstalled extensions), a warning is logged and the flow
+        is returned without instantiating the model.
     strict_version : bool, optional (default=True)
         When ``reinstantiate`` is True, whether to enforce exact version
         requirements for the extension/model. If False, a new flow may
@@ -95,7 +98,7 @@ def get_flow(flow_id: int, reinstantiate: bool = False, strict_version: bool = T
     -------
     OpenMLFlow
         The flow object with metadata; ``model`` may be populated when
-        ``reinstantiate=True``.
+        ``reinstantiate=True`` and an extension is available.
 
     Raises
     ------
@@ -108,6 +111,8 @@ def get_flow(flow_id: int, reinstantiate: bool = False, strict_version: bool = T
     ------------
     - Writes to ``openml.config.cache_directory/flows/{flow_id}/flow.xml``
       when the flow is downloaded from the server.
+    - Logs a warning if ``reinstantiate=True`` but no extension is available
+      for the flow type.
 
     Preconditions
     -------------
@@ -124,17 +129,35 @@ def get_flow(flow_id: int, reinstantiate: bool = False, strict_version: bool = T
     --------
     >>> import openml
     >>> flow = openml.flows.get_flow(5)  # doctest: +SKIP
+    >>> # Reinstantiate a sklearn flow (requires openml-sklearn extension)
+    >>> flow_with_model = openml.flows.get_flow(5, reinstantiate=True)  # doctest: +SKIP
+    >>> # For flows without extensions, a warning is logged and flow.model remains None
+    >>> flow_no_model = openml.flows.get_flow(1, reinstantiate=True)  # doctest: +SKIP
     """
     flow_id = int(flow_id)
     flow = _get_flow_description(flow_id)
 
     if reinstantiate:
-        flow.model = flow.extension.flow_to_model(flow, strict_version=strict_version)
-        if not strict_version:
-            # check if we need to return a new flow b/c of version mismatch
-            new_flow = flow.extension.model_to_flow(flow.model)
-            if new_flow.dependencies != flow.dependencies:
-                return new_flow
+        # Try to get an extension that can handle this flow
+        extension = flow._extension
+        if extension is None:
+            extension = openml.extensions.get_extension_by_flow(flow, raise_if_no_extension=False)
+
+        if extension is not None:
+            flow.model = extension.flow_to_model(flow, strict_version=strict_version)
+            if not strict_version:
+                # check if we need to return a new flow b/c of version mismatch
+                new_flow = extension.model_to_flow(flow.model)
+                if new_flow.dependencies != flow.dependencies:
+                    return new_flow
+        else:
+            # No extension available for this flow type, log a warning
+            logger = logging.getLogger(__name__)
+            logger.warning(
+                f"Cannot reinstantiate flow {flow_id} ({flow.name}): "
+                f"no extension registered that can handle this flow type. "
+                f"Returning flow without model instance."
+            )
     return flow
 
 

diff --git a/tests/test_flows/test_flow_functions.py b/tests/test_flows/test_flow_functions.py
@@ -324,14 +324,39 @@ def test_get_flow_reinstantiate_model(self):
 
     @pytest.mark.test_server()
     def test_get_flow_reinstantiate_model_no_extension(self):
-        # Flow 10 is a WEKA flow
-        self.assertRaisesRegex(
-            ValueError,
-            ".* flow: 10 \(weka.SMO\). ",
-            openml.flows.get_flow,
-            flow_id=10,
-            reinstantiate=True,
-        )
+        # Flow 10 is a WEKA flow without a registered extension
+        # Should not raise an error, but log a warning and return flow without model
+        with self.assertLogs("openml.flows.functions", level="WARNING") as cm:
+            flow = openml.flows.get_flow(flow_id=10, reinstantiate=True)
+
+        # Verify that a warning was logged
+        assert any("Cannot reinstantiate flow" in log for log in cm.output)
+        assert any("no extension registered" in log for log in cm.output)
+
+        # Verify that the flow is returned but without a model
+        assert flow is not None
+        assert flow.flow_id == 10
+        assert flow.model is None
+
+    @pytest.mark.test_server()
+    def test_get_flow_reinstantiate_openml_native_flow(self):
+        """Test for issue #1626: get_flow() with reinstantiate=True for OpenML-native flows.
+
+        OpenML-native flows (e.g., weka.ZeroR on test server) don't have
+        registered extensions. When reinstantiate=True is passed, the function should
+        log a warning and return the flow without raising an error.
+        """
+        with self.assertLogs("openml.flows.functions", level="WARNING") as cm:
+            flow = openml.flows.get_flow(flow_id=1, reinstantiate=True)
+
+        # Verify that a warning was logged
+        assert any("Cannot reinstantiate flow 1" in log for log in cm.output)
+        assert any("no extension registered" in log for log in cm.output)
+
+        # Verify that the flow is returned but without a model
+        assert flow is not None
+        assert flow.flow_id == 1
+        assert flow.model is None
 
     @pytest.mark.sklearn()
     @unittest.skipIf(