From b473e667ff76cef68334b5cce0e15b0810a56c65 Mon Sep 17 00:00:00 2001
From: Lucas <119979961+lucas-flexcompute@users.noreply.github.com>
Date: Tue, 10 Oct 2023 12:58:32 -0300
Subject: [PATCH] Issue a warning when accessing unavailable group index data
 (#1169) (#1178)

Signed-off-by: Lucas Heitzmann Gabrielli <lucas@flexcompute.com>
---
 tests/test_plugins/test_mode_solver.py |  7 ++++++-
 tidy3d/components/data/dataset.py      | 14 +++++++++++++-
 tidy3d/components/data/monitor_data.py | 18 +++++++++++++++---
 3 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/tests/test_plugins/test_mode_solver.py b/tests/test_plugins/test_mode_solver.py
index 2cd9f6c66..c3b04091a 100644
--- a/tests/test_plugins/test_mode_solver.py
+++ b/tests/test_plugins/test_mode_solver.py
@@ -531,7 +531,7 @@ def test_mode_solver_2D():
 
 @pytest.mark.parametrize("local", [True, False])
 @responses.activate
-def test_group_index(mock_remote_api, local):
+def test_group_index(mock_remote_api, log_capture, local):
     """Test group index calculation"""
 
     simulation = td.Simulation(
@@ -570,6 +570,11 @@ def test_group_index(mock_remote_api, local):
     modes = ms.solve() if local else msweb.run(ms)
     if local:
         assert modes.n_group is None
+        assert len(log_capture) == 1
+        assert log_capture[0][0] == 30
+        assert "ModeSpec" in log_capture[0][1]
+        _ = modes.n_group
+        assert len(log_capture) == 1
 
     # Group index calculated
     ms = ModeSolver(
diff --git a/tidy3d/components/data/dataset.py b/tidy3d/components/data/dataset.py
index b5f4fa418..81a0ff5fa 100644
--- a/tidy3d/components/data/dataset.py
+++ b/tidy3d/components/data/dataset.py
@@ -329,8 +329,9 @@ class ModeSolverDataset(ElectromagneticFieldDataset):
         description="Complex-valued effective propagation constants associated with the mode.",
     )
 
-    n_group: ModeIndexDataArray = pd.Field(
+    n_group_raw: ModeIndexDataArray = pd.Field(
         None,
+        alias="n_group",
         title="Group Index",
         description="Index associated with group velocity of the mode.",
     )
@@ -351,6 +352,17 @@ def k_eff(self):
         """Imaginary part of the propagation index."""
         return self.n_complex.imag
 
+    @property
+    def n_group(self):
+        """Group index."""
+        if self.n_group_raw is None:
+            log.warning(
+                "The group index was not computed. To calculate group index, pass "
+                "'group_index_step = True' in the 'ModeSpec'.",
+                log_once=True,
+            )
+        return self.n_group_raw
+
     def plot_field(self, *args, **kwargs):
         """Warn user to use the :class:`.ModeSolver` ``plot_field`` function now."""
         raise DeprecationWarning(
diff --git a/tidy3d/components/data/monitor_data.py b/tidy3d/components/data/monitor_data.py
index b30cf4250..eb0b0a95a 100644
--- a/tidy3d/components/data/monitor_data.py
+++ b/tidy3d/components/data/monitor_data.py
@@ -1179,7 +1179,7 @@ def _group_index_post_process(self, frequency_step: float) -> ModeSolverData:
         )
 
         # remove data corresponding to frequencies used only for group index calculation
-        update_dict = {"n_complex": self.n_complex.isel(f=center), "n_group": n_group}
+        update_dict = {"n_complex": self.n_complex.isel(f=center), "n_group_raw": n_group}
 
         for key, field in self.field_components.items():
             update_dict[key] = field.isel(f=center)
@@ -1313,7 +1313,7 @@ def modes_info(self) -> xr.Dataset:
             "wg TE fraction": self.pol_fraction_waveguide["te"],
             "wg TM fraction": self.pol_fraction_waveguide["tm"],
             "mode area": self.mode_area,
-            "group index": self.n_group,
+            "group index": self.n_group_raw,  # Use raw field to avoid issuing a warning
         }
 
         return xr.Dataset(data_vars=info)
@@ -1394,8 +1394,9 @@ class ModeData(MonitorData):
         description="Complex-valued effective propagation constants associated with the mode.",
     )
 
-    n_group: ModeIndexDataArray = pd.Field(
+    n_group_raw: ModeIndexDataArray = pd.Field(
         None,
+        alias="n_group",
         title="Group Index",
         description="Index associated with group velocity of the mode.",
     )
@@ -1410,6 +1411,17 @@ def k_eff(self):
         """Imaginary part of the propagation index."""
         return self.n_complex.imag
 
+    @property
+    def n_group(self):
+        """Group index."""
+        if self.n_group_raw is None:
+            log.warning(
+                "The group index was not computed. To calculate group index, pass "
+                "'group_index_step = True' in the 'ModeSpec'.",
+                log_once=True,
+            )
+        return self.n_group_raw
+
     def normalize(self, source_spectrum_fn) -> ModeData:
         """Return copy of self after normalization is applied using source spectrum function."""
         source_freq_amps = source_spectrum_fn(self.amps.f)[None, :, None]