fix: callables can be deserialized from fully qualified import path (#8788)

* fix: callables can be deserialized from fully qualified import path

* fix: license header

* fix: format

* fix: types

* fix? types

* test: extend test case

* format

* add release notes

Author: mathislucka

Diff for: haystack/testing/callable_serialization/random_callable.py

@@ -0,0 +1,10 @@
+# SPDX-FileCopyrightText: 2022-present deepset GmbH <[email protected]>
+#
+# SPDX-License-Identifier: Apache-2.0
+
+
+def callable_to_deserialize(hello: str) -> str:
+    """
+    A function to test callable deserialization.
+    """
+    return f"{hello}, world!"

Diff for: haystack/utils/callable_serialization.py

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 import inspect
-from typing import Callable
+from typing import Any, Callable
 
 from haystack.core.errors import DeserializationError, SerializationError
 from haystack.utils.type_serialization import thread_safe_import
@@ -50,26 +50,31 @@ def deserialize_callable(callable_handle: str) -> Callable:
     :return: The callable
     :raises DeserializationError: If the callable cannot be found
     """
-    module_name, *attribute_chain = callable_handle.split(".")
+    parts = callable_handle.split(".")
 
-    try:
-        current = thread_safe_import(module_name)
-    except Exception as e:
-        raise DeserializationError(f"Could not locate the module: {module_name}") from e
-
-    for attr in attribute_chain:
+    for i in range(len(parts), 0, -1):
+        module_name = ".".join(parts[:i])
         try:
-            attr_value = getattr(current, attr)
-        except AttributeError as e:
-            raise DeserializationError(f"Could not find attribute '{attr}' in {current.__name__}") from e
+            mod: Any = thread_safe_import(module_name)
+        except Exception:
+            # keep reducing i until we find a valid module import
+            continue
+
+        attr_value = mod
+        for part in parts[i:]:
+            try:
+                attr_value = getattr(attr_value, part)
+            except AttributeError as e:
+                raise DeserializationError(f"Could not find attribute '{part}' in {attr_value.__name__}") from e
 
         # when the attribute is a classmethod, we need the underlying function
         if isinstance(attr_value, (classmethod, staticmethod)):
             attr_value = attr_value.__func__
 
-        current = attr_value
+        if not callable(attr_value):
+            raise DeserializationError(f"The final attribute is not callable: {attr_value}")
 
-    if not callable(current):
-        raise DeserializationError(f"The final attribute is not callable: {current}")
+        return attr_value
 
-    return current
+    # Fallback if we never find anything
+    raise DeserializationError(f"Could not import '{callable_handle}' as a module or callable.")

Diff for: releasenotes/notes/fix-callable-deserialization-5a3ef204a8d07616.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Callable deserialization now works for all fully qualified import paths.

Diff for: test/utils/test_callable_serialization.py

@@ -1,10 +1,12 @@
 # SPDX-FileCopyrightText: 2022-present deepset GmbH <[email protected]>
 #
 # SPDX-License-Identifier: Apache-2.0
+
 import pytest
 import requests
 from haystack.core.errors import DeserializationError, SerializationError
 from haystack.components.generators.utils import print_streaming_chunk
+from haystack.testing.callable_serialization.random_callable import callable_to_deserialize
 from haystack.utils import serialize_callable, deserialize_callable
 
 
@@ -40,6 +42,13 @@ def test_callable_serialization_non_local():
     assert result == "requests.api.get"
 
 
+def test_fully_qualified_import_deserialization():
+    func = deserialize_callable("haystack.testing.callable_serialization.random_callable.callable_to_deserialize")
+
+    assert func is callable_to_deserialize
+    assert func("Hello") == "Hello, world!"
+
+
 def test_callable_serialization_instance_methods_fail():
     with pytest.raises(SerializationError):
         serialize_callable(TestClass.my_method)