Improve error message when trying to unpack a step artifact

src/zenml/steps/entrypoint_function_utils.py

@@ -20,6 +20,7 @@
     Callable,
     Dict,
     NamedTuple,
+    NoReturn,
     Optional,
     Sequence,
     Type,
@@ -103,6 +104,20 @@ def __init__(
         self.annotation = annotation
         self.pipeline = pipeline
 
+    def __iter__(self) -> NoReturn:
+        """Raise a custom error if someone is trying to iterate this object.
+
+        Raises:
+            StepInterfaceError: If trying to iterate this object.
+        """
+        raise StepInterfaceError(
+            "Unable to unpack step artifact. This error is probably because "
+            "you're trying to unpack the return value of your step but the "
+            "step only returns a single artifact. For more information on how "
+            "to add type annotations to your step to indicate multiple "
+            "artifacts visit https://docs.zenml.io/user-guide/advanced-guide/pipelining-features/managing-steps#type-annotations."
+        )
+
 
 def validate_reserved_arguments(
     signature: inspect.Signature, reserved_arguments: Sequence[str]

tests/unit/steps/test_base_step_new.py

@@ -170,3 +170,20 @@ def test_pipeline():
 
     with does_not_raise():
         test_pipeline()
+
+
+@step
+def step_with_single_output() -> int:
+    return 1
+
+
+def test_unpacking_step_artifact_raises_custom_exception():
+    """Tests that unpacking an artifact returned by a step inside a pipeline
+    raises a custom exception with explanation on how to solve the issue."""
+
+    @pipeline
+    def test_pipeline():
+        a, b = step_with_single_output()
+
+    with pytest.raises(StepInterfaceError):
+        test_pipeline()