Allow None return annotation on task execute (#42)

Author: snamber

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- `tilebox-workflows`: Allow task `execute()` methods to use a `-> None` return annotation when postponed annotation
+  evaluation is enabled.
+
 ## [0.55.0] - 2026-07-01
 
 ### Added

tilebox-workflows/tests/test_task.py

@@ -1,3 +1,5 @@
+import __future__
+
 import json
 from dataclasses import dataclass
 from typing import Annotated
@@ -46,6 +48,35 @@ def execute(self, context: ExecutionContext) -> None:
     assert TaskMeta.for_task(SimpleTask).executable is True
 
 
+def _compile_task_with_postponed_return_annotation(return_annotation: str) -> type:
+    source = f"""
+class PostponedAnnotationsTask(Task):
+    def execute(self, context: ExecutionContext) -> {return_annotation}:
+        pass
+"""
+    namespace: dict[str, type] = {"Task": Task, "ExecutionContext": ExecutionContext}
+    code = compile(
+        source,
+        filename="<postponed-annotations-test>",
+        mode="exec",
+        flags=__future__.annotations.compiler_flag,
+        dont_inherit=True,
+    )
+    exec(code, namespace)  # noqa: S102
+    return namespace["PostponedAnnotationsTask"]
+
+
+def test_task_validation_execute_none_return_type_with_postponed_annotations() -> None:
+    task_class = _compile_task_with_postponed_return_annotation("None")
+
+    assert TaskMeta.for_task(task_class).executable is True
+
+
+def test_task_validation_execute_invalid_return_type_with_postponed_annotations() -> None:
+    with pytest.raises(TypeError, match="to not have a return value"):
+        _compile_task_with_postponed_return_annotation("int")
+
+
 def test_task_validation_execute_invalid_signature_no_params() -> None:
     with pytest.raises(TypeError, match="Expected a function signature of"):
         # validation happens at class creation time, that's why we create it in a function

tilebox-workflows/tilebox/workflows/task.py

@@ -136,7 +136,8 @@ def _validate_execute_method(
             f"but got {class_name}.execute{signature}!"
         )
 
-    if signature.return_annotation is not None and signature.return_annotation != inspect._empty:  # noqa: SLF001
+    # `from __future__ import annotations` stores `-> None` as the string "None".
+    if signature.return_annotation not in (None, "None", inspect.Signature.empty):
         raise TypeError(f"Expected {class_name}.execute{signature} to not have a return value!")
 
     return True