Fix: Add pickle support for UnwrapFailedError (#2048)

CHANGELOG.md

@@ -21,6 +21,9 @@ See [0Ver](https://0ver.org/).
 - Add `default_error` parameter to `returns.converters.maybe_to_result`,
   which provides a default error value for `Failure`
 
+### Bugfixes
+
+- Add pickling support for `UnwrapFailedError` exception
 
 ## 0.23.0
 

returns/primitives/exceptions.py

@@ -21,6 +21,16 @@ def __init__(self, container: Unwrappable) -> None:
         super().__init__()
         self.halted_container = container
 
+    def __reduce__(self):  # noqa: WPS603
+        """Custom reduce method for pickle protocol.
+
+        This helps properly reconstruct the exception during unpickling.
+        """
+        return (
+            self.__class__,  # callable
+            (self.halted_container,),  # args to callable
+        )
+
 
 class ImmutableStateError(AttributeError):
     """

tests/test_primitives/test_exceptions/test_pickle_unwrap_failed_error.py

@@ -0,0 +1,37 @@
+import pickle  # noqa: S403
+
+from returns.maybe import Nothing, Some
+from returns.primitives.exceptions import UnwrapFailedError
+from returns.result import Failure, Success
+
+
+def test_pickle_unwrap_failed_error_from_maybe():
+    """Ensures that UnwrapFailedError with Maybe can be pickled."""
+    try:
+        Nothing.unwrap()  # This will raise UnwrapFailedError
+    except UnwrapFailedError as error:
+        # Serialize the error
+        serialized = pickle.dumps(error)
+
+        # Deserialize
+        deserialized_error = pickle.loads(serialized)  # noqa: S301
+
+        # Check that halted_container is preserved
+        assert deserialized_error.halted_container == Nothing
+        assert deserialized_error.halted_container != Some(None)
+
+
+def test_pickle_unwrap_failed_error_from_result():
+    """Ensures that UnwrapFailedError with Result can be pickled."""
+    try:
+        Failure('error').unwrap()  # This will raise UnwrapFailedError
+    except UnwrapFailedError as error:
+        # Serialize the error
+        serialized = pickle.dumps(error)
+
+        # Deserialize
+        deserialized_error = pickle.loads(serialized)  # noqa: S301
+
+        # Check that halted_container is preserved
+        assert deserialized_error.halted_container == Failure('error')
+        assert deserialized_error.halted_container != Success('error')