fix(tidy3d): fix empty dict deserialization for NonLinearSpec

Author: marcorudolphflex

changelog.d/3329.fixed.md

@@ -0,0 +1 @@
+Fixed loading of medium settings so empty `NonlinearSpec` inputs are ignored instead of unintentionally enabling nonlinearity.

tests/test_components/test_medium.py

@@ -791,6 +791,15 @@ def test_nonlinear_medium():
         _ = sim.updated_copy(medium=med, path="structures/0")
 
 
+def test_nonlinear_spec_empty_dict_is_ignored():
+    assert td.Medium(nonlinear_spec=td.NonlinearSpec()).nonlinear_spec is not None
+
+    medium_dict = td.Medium().model_dump()
+    medium_dict["nonlinear_spec"] = {}
+
+    assert td.Medium.model_validate(medium_dict).nonlinear_spec is None
+
+
 def test_custom_medium():
     Nx, Ny, Nz, Nf = 4, 3, 1, 1
     X = np.linspace(-1, 1, Nx)

tidy3d/components/medium.py

@@ -4,7 +4,7 @@
 
 import functools
 from abc import ABC, abstractmethod
-from collections.abc import Sequence
+from collections.abc import Mapping, Sequence
 from math import isclose
 from typing import TYPE_CHECKING, Any, Callable, Literal, Optional, TypeVar, Union, get_args
 
@@ -244,6 +244,14 @@ def _run_after_validators(self) -> Self:
         self._validate_modulation_spec_after()
         return self
 
+    @field_validator("nonlinear_spec", mode="before")
+    @classmethod
+    def _normalize_empty_nonlinear_spec_dict(cls, val: Any) -> Any:
+        """Treat an empty nonlinear spec mapping as a missing value."""
+        if isinstance(val, Mapping) and not val:
+            return None
+        return val
+
     def _validate_nonlinear_spec(self) -> Self:
         """Check compatibility with nonlinear_spec."""
         if self.__class__.__name__ == "AnisotropicMedium" and any(