fix: serialize NaN values as JSON null

src/power_grid_model_ds/_core/model/grids/serialization/json.py

@@ -7,6 +7,7 @@
 import dataclasses
 import json
 import logging
+import math
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
@@ -64,7 +65,7 @@ def serialize_to_dict[G: Grid](grid: G, strict: bool = True, **kwargs) -> dict:
         if _is_serializable(field_value, strict, **kwargs):
             serialized_data[field.name] = field_value
 
-    return {"data": serialized_data}
+    return {"data": _replace_nan_with_none(serialized_data)}
 
 
 def deserialize_from_json[G: Grid](path: Path, target_grid_class: type[G]) -> G:
@@ -182,3 +183,14 @@ def _is_serializable(value: Any, strict: bool, **kwargs) -> bool:
         _logger.warning(msg)
         return False
     return True
+
+
+def _replace_nan_with_none(value: Any) -> Any:
+    """Recursively replace NaN values with JSON-compatible null values."""
+    if isinstance(value, float) and math.isnan(value):
+        return None
+    if isinstance(value, dict):
+        return {key: _replace_nan_with_none(item) for key, item in value.items()}
+    if isinstance(value, list):
+        return [_replace_nan_with_none(item) for item in value]
+    return value

tests/unit/model/grids/serialization/data/legacy_nan.json

@@ -0,0 +1,10 @@
+{
+  "data": {
+    "node": [
+      {
+        "id": 1,
+        "u_rated": NaN
+      }
+    ]
+  }
+}

tests/unit/model/grids/serialization/data/legacy_nan.json.license

@@ -0,0 +1,3 @@
+SPDX-FileCopyrightText: Contributors to the Power Grid Model project <powergridmodel@lfenergy.org>
+
+SPDX-License-Identifier: MPL-2.0

tests/unit/model/grids/serialization/test_json.py

@@ -155,6 +155,25 @@ def test_pgm_roundtrip(self, request, grid_fixture: str, tmp_path: Path):
 
             assert array_equal_with_nan(original_array, loaded_array), f"Array '{array_name}' does not match"
 
+    def test_nan_serializes_as_json_null(self, basic_grid: Grid, tmp_path: Path):
+        basic_grid.node.u_rated[0] = np.nan
+
+        path = basic_grid.serialize(tmp_path / "grid.json")
+        with path.open(encoding="utf-8") as file:
+            file_data = json.load(file)
+
+        string_data = json.loads(basic_grid.serialize(mode="json_string"))
+
+        assert file_data["data"]["node"][0]["u_rated"] is None
+        assert string_data["data"]["node"][0]["u_rated"] is None
+
+    def test_deserialize_legacy_nan_json(self):
+        path = Path(__file__).parent / "data" / "legacy_nan.json"
+
+        grid = Grid.deserialize(path)
+
+        assert np.isnan(grid.node.u_rated[0])
+
 
 class TestCrossTypeCompatibility:
     """Test cross-type loading and compatibility"""