fix(util): keep None as null in to_pod instead of the string "None"

metaflow/util.py

@@ -573,6 +573,8 @@ def to_pod(value):
     # Prevent circular imports
     from metaflow.parameters import DeployTimeField
 
+    if value is None:
+        return value
     if isinstance(value, (str, int, float)):
         return value
     if isinstance(value, dict):

test/unit/test_graph_structure.py

@@ -693,3 +693,31 @@ def test_step_mutator_non_primitive_attribute_sanitized_in_output_steps():
     decorators = steps_info["only"]["decorators"]
     payload_deco = next(d for d in decorators if d["name"].endswith("_payload_mutator"))
     assert isinstance(payload_deco["attributes"]["payload"], str)
+
+
+# ---------------------------------------------------------------------------
+# Tests: unset decorator attributes stay null in _graph_info
+# ---------------------------------------------------------------------------
+
+
+class _PartialResourcesFlow(FlowSpec):
+    @resources(cpu=2, memory=8192)
+    @step(start=True, end=True)
+    def only(self):
+        pass
+
+
+def test_unset_decorator_attributes_stay_null_in_output_steps():
+    """@resources defaults gpu/disk/shared_memory to None. output_steps() runs
+    the attribute dict through to_pod(), which must leave None alone: the
+    string "None" is truthy, so a consumer of _graph_info (the DAG card, the
+    UI, `current.graph`) reading attributes["gpu"] would conclude a GPU was
+    requested when the user never asked for one."""
+    steps_info, _ = _PartialResourcesFlow._graph.output_steps()
+    decorators = steps_info["only"]["decorators"]
+    attrs = next(d for d in decorators if d["name"] == "resources")["attributes"]
+    assert attrs["cpu"] == 2
+    assert attrs["memory"] == 8192
+    assert attrs["gpu"] is None
+    assert attrs["disk"] is None
+    assert attrs["shared_memory"] is None

test/unit/test_to_pod.py

@@ -62,3 +62,20 @@ def test_to_pod_lambda_uses_qualname():
     fn = lambda x: x  # noqa: E731
     result = to_pod(fn)
     assert "<lambda>" in result
+
+
+def test_to_pod_none_is_preserved():
+    """None is already POD (JSON null) and must not be stringified."""
+    assert to_pod(None) is None
+
+
+def test_to_pod_none_nested_in_dict_and_list():
+    """Unset decorator attributes are None; they must stay null in _graph_info."""
+    assert to_pod({"gpu": None, "cpu": 1}) == {"gpu": None, "cpu": 1}
+    assert to_pod([1, None, "x"]) == [1, None, "x"]
+    assert to_pod({"outer": {"inner": [None]}}) == {"outer": {"inner": [None]}}
+
+
+def test_to_pod_none_is_falsy_after_conversion():
+    """The string "None" is truthy; callers testing the attribute must not be misled."""
+    assert not to_pod({"gpu": None})["gpu"]