Raise error for invalid value in return annotation.

Author: tobiasraabe

docs/source/changes.md

@@ -13,6 +13,7 @@ releases are available on [PyPI](https://pypi.org/project/pytask) and
 - {pull}`485` adds missing steps to unconfigure pytask after the job is done which
   caused flaky tests.
 - {pull}`486` adds default names to {class}`~pytask.PPathNode`.
+- {pull}`488` raises an error when an invalid value is used in a return annotation.
 
 ## 0.4.2 - 2023-11-8
 

src/_pytask/collect.py

@@ -308,7 +308,7 @@ def pytask_collect_task(
 
 
 @hookimpl(trylast=True)
-def pytask_collect_node(session: Session, path: Path, node_info: NodeInfo) -> PNode:
+def pytask_collect_node(session: Session, path: Path, node_info: NodeInfo) -> PNode:  # noqa: C901
     """Collect a node of a task as a :class:`pytask.PNode`.
 
     Strings are assumed to be paths. This might be a strict assumption, but since this
@@ -375,6 +375,18 @@ def pytask_collect_node(session: Session, path: Path, node_info: NodeInfo) -> PN
 
         return PathNode(name=name, path=node)
 
+    # Allowing a PythonNode as a return is a poor fallback, because it cannot be used.
+    # Probably, the user made a mistake like writing a custom node that does not
+    # strictly follow the protocol or some other misspecification.
+    if node_info.arg_name == "return":
+        msg = (
+            "The return annotation of the task holds an invalid value. Please, use a "
+            "node or a value that can be parsed to a node. Maybe you used a node that "
+            "does not follow the 'pytask.PNode' protocol. This is the value: "
+            f"{node_info.value!r}"
+        )
+        raise ValueError(msg)
+
     node_name = create_name_of_python_node(node_info)
     return PythonNode(value=node, name=node_name, node_info=node_info)
 

tests/test_collect.py

@@ -641,3 +641,16 @@ def task_example() -> Annotated[str, {node}]:
     assert session.exit_code == ExitCode.OK
     assert tmp_path.joinpath("file.txt").exists()
     assert session.tasks[0].produces["return"].name == tmp_path.name + "/file.txt"
+
+
+def test_error_when_return_annotation_cannot_be_parsed(runner, tmp_path):
+    source = """
+    from typing_extensions import Annotated
+
+    def task_example() -> Annotated[int, 1]: ...
+    """
+    tmp_path.joinpath("task_example.py").write_text(textwrap.dedent(source))
+
+    result = runner.invoke(cli, [tmp_path.as_posix()])
+    assert result.exit_code == ExitCode.COLLECTION_FAILED
+    assert "The return annotation of the task" in result.output