Fixes for group_index_step validator (#1143)

Signed-off-by: Lucas Heitzmann Gabrielli <lucas@flexcompute.com>

CHANGELOG.md

@@ -4,6 +4,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+### Changed
+### Fixed
+- Validation for `ModeSpec.group_index_step` working incorrectly if the value was set to 1.
 
 ## [2.4.0] - 2023-9-11
 

tests/test_components/test_mode.py

@@ -28,3 +28,14 @@ def test_bend_axis_not_given():
 def test_glancing_incidence():
     with pytest.raises(pydantic.ValidationError):
         _ = td.ModeSpec(angle_theta=np.pi / 2)
+
+def test_group_index_step_validation():
+    with pytest.raises(pydantic.ValidationError):
+        _ = td.ModeSpec(group_index_step=1.0)
+
+    ms = td.ModeSpec(group_index_step=True)
+    assert ms.group_index_step == td.components.mode.GROUP_INDEX_STEP
+
+    ms = td.ModeSpec(group_index_step=False)
+    assert ms.group_index_step is False
+    assert not ms.group_index_step > 0

tidy3d/components/mode.py

@@ -104,7 +104,7 @@ class ModeSpec(Tidy3dBaseModel):
         "If ``None`` no mode tracking is performed.",
     )
 
-    group_index_step: Union[bool, pd.PositiveFloat] = pd.Field(
+    group_index_step: Union[pd.PositiveFloat, bool] = pd.Field(
         False,
         title="Frequency step for group index computation",
         description="Control the computation of the group index alongside the effective index. If "
@@ -130,16 +130,24 @@ def glancing_incidence(cls, val):
             )
         return val
 
-    @pd.validator("group_index_step")
+    # Must be executed before type validation by pydantic, otherwise True is converted to 1.0
+    @pd.validator("group_index_step", pre=True)
     def assign_default_on_true(cls, val):
-        """Assing the default fractional frequency step value if not provided."""
+        """Assign the default fractional frequency step value if not provided."""
         if val is True:
             return GROUP_INDEX_STEP
+        return val
+
+    @pd.validator("group_index_step")
+    def check_group_step_size(cls, val):
+        """Ensure a reasonable group index step is used."""
         if val >= 1:
-            raise ValidationError("Parameter 'group_index_step' must be less than 1.")
+            raise ValidationError(
+                "Parameter 'group_index_step' is a fractional value. It must be less than 1."
+            )
         return val
 
-    @pd.root_validator()
+    @pd.root_validator(skip_on_failure=True)
     def check_precision(cls, values):
         """Verify critical ModeSpec settings for group index calculation."""
         if values["group_index_step"] > 0: