From c8b64a7ae44ce221754fc2bcd5b894090f56b505 Mon Sep 17 00:00:00 2001
From: kevin9327 <5299031+kevin9327@users.noreply.github.com>
Date: Thu, 10 Sep 2026 22:49:09 +0900
Subject: [PATCH] fix(util): keep None as null in to_pod instead of the string
 "None"

to_pod() passes str/int/float/bool through unchanged but fell through to
str(value) for None, turning it into the string "None".

output_steps() and FlowSpec._graph_info run every decorator's attribute
dict through to_pod(), and most decorators default their optional
attributes to None. @resources(cpu=2, memory=8192) therefore serialized
gpu/disk/shared_memory as "None" rather than null. "None" is a truthy
string, so a consumer of _graph_info reading attributes["gpu"] concludes
a GPU was requested when the user never asked for one.

None is already plain-old-data, so returning it unchanged is consistent
with the other JSON-native scalars the function already passes through.
---
 metaflow/util.py                  |  2 ++
 test/unit/test_graph_structure.py | 28 ++++++++++++++++++++++++++++
 test/unit/test_to_pod.py          | 17 +++++++++++++++++
 3 files changed, 47 insertions(+)

diff --git a/metaflow/util.py b/metaflow/util.py
index 4cc96d8b0cf..9f849a4af89 100644
--- a/metaflow/util.py
+++ b/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):
diff --git a/test/unit/test_graph_structure.py b/test/unit/test_graph_structure.py
index 325985ee161..d7e3c0a27ad 100644
--- a/test/unit/test_graph_structure.py
+++ b/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
diff --git a/test/unit/test_to_pod.py b/test/unit/test_to_pod.py
index 3968dc1fbbe..98ce965b668 100644
--- a/test/unit/test_to_pod.py
+++ b/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"]