BUG: serialize numpy SeedSequence for sensor seeds (#1087)

rocketpy/_encoders.py

@@ -56,6 +56,17 @@ def default(self, o):
             return o.item()
         elif isinstance(o, np.ndarray):
             return o.tolist()
+        elif isinstance(o, np.random.SeedSequence):
+            # Sensor seeds (and other RNGs) may hold a SeedSequence. Encode its
+            # reconstructible state so JSON dump does not raise TypeError.
+            encoding = {
+                "entropy": o.entropy,
+                "spawn_key": list(o.spawn_key),
+                "n_children_spawned": int(o.n_children_spawned),
+                "pool_size": int(o.pool_size),
+            }
+            encoding["signature"] = get_class_signature(o)
+            return encoding
         elif isinstance(o, datetime):
             return [o.year, o.month, o.day, o.hour]
         elif hasattr(o, "__iter__") and not isinstance(o, str):
@@ -110,6 +121,15 @@ def object_hook(self, obj):
                 class_ = get_class_from_signature(signature)
                 hash_ = signature.get("hash", None)
 
+                if class_ is np.random.SeedSequence:
+                    # Cython __init__ has no __code__, so the generic kwargs
+                    # path cannot rebuild SeedSequence; restore from state.
+                    return np.random.SeedSequence(
+                        entropy=obj.get("entropy"),
+                        spawn_key=tuple(obj.get("spawn_key", ())),
+                        pool_size=obj.get("pool_size", 4),
+                        n_children_spawned=obj.get("n_children_spawned", 0),
+                    )
                 if class_.__name__ == "Flight" and not self.resimulate:
                     new_flight = class_.__new__(class_)
                     new_flight.prints = _FlightPrints(new_flight)

tests/unit/sensors/test_sensor_seeding.py

@@ -16,7 +16,7 @@
 
 import numpy as np
 
-from rocketpy._encoders import RocketPyEncoder
+from rocketpy._encoders import RocketPyDecoder, RocketPyEncoder
 from rocketpy.mathutils.vector_matrix import Vector
 from rocketpy.sensors.accelerometer import Accelerometer
 from rocketpy.sensors.barometer import Barometer
@@ -139,3 +139,17 @@ def test_from_dict_defaults_seed_to_none_when_absent():
     ).to_dict()
     del data["seed"]
     assert GnssReceiver.from_dict(data).to_dict()["seed"] is None
+
+
+def test_seedsequence_sensor_seed_is_json_serializable():
+    """SeedSequence seeds must serialize through RocketPyEncoder (#1087)."""
+    seed = np.random.SeedSequence(0).spawn(1)[0]
+    sensor = Accelerometer(sampling_rate=100, seed=seed)
+
+    encoded = json.dumps(sensor.to_dict(), cls=RocketPyEncoder)
+    decoded = json.loads(encoded, cls=RocketPyDecoder)
+
+    assert isinstance(decoded["seed"], np.random.SeedSequence)
+    assert decoded["seed"].state == seed.state
+    restored = Accelerometer.from_dict(decoded)
+    assert restored.to_dict()["seed"].state == seed.state