Avoid error in importing ModeSolver if scipy is not installed

Author: momchil-flex

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Warning if nonlinear mediums are used in an `adjoint` simulation. In this case, the gradients will not be accurate, but may be approximately correct if the nonlinearity is weak.
 
 ### Changed
+- Credit cost for remote mode solver has been modified to be defined in advance based on the mode solver details. Previously, the cost was based on elapsed runtime. On average, there should be little difference in the cost.
+- Mode solves that are part of an FDTD simulation (i.e. for mode sources and monitors) are now charged at the same flex credit cost as a corresponding standalone mode solver call.
 
 ### Fixed
 
@@ -34,8 +36,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Removed spurious ``-1`` factor in field amplitudes injected by field sources in some cases. The injected ``E``-field should now exactly match the analytic, mode, or custom fields that the source is expected to inject, both in the forward and in the backward direction.
 - Restriction on the maximum memory that a monitor would need internally during the solver run, even if the final monitor data is smaller.
 - Restriction on the maximum size of mode solver data produced by a `ModeSolver` server call.
-- Credit cost for remote mode solver has been modified to be defined in advance based on the mode solver details. Previously, the cost was based on elapses runtime. On average, there should be little difference in the cost.
-- Mode solves that are part of an FDTD simulation (i.e. for mode sources and monitors) are now charged at the same flex credit cost as a corresponding standalone mode solver call.
 
 ### Fixed
 - Fixed the duplication of log messages in Jupyter when `set_logging_file` is used.

tidy3d/plugins/mode/mode_solver.py

@@ -26,7 +26,18 @@
 from ...components.data.monitor_data import ModeSolverData
 from ...exceptions import ValidationError, SetupError
 from ...constants import C_0
-from .solver import compute_modes, EigSolver
+
+# Importing the local solver may not work if e.g. scipy is not installed
+IMPORT_ERROR_MSG = """Could not import local solver, 'ModeSolver' objects can still be constructed
+but will have to be run through the server.
+"""
+try:
+    from .solver import compute_modes
+
+    LOCAL_SOLVER_IMPORTED = True
+except ImportError:
+    log.warning(IMPORT_ERROR_MSG)
+    LOCAL_SOLVER_IMPORTED = False
 
 FIELD = Tuple[ArrayComplex3D, ArrayComplex3D, ArrayComplex3D]
 MODE_MONITOR_NAME = "<<<MODE_SOLVER_MONITOR>>>"
@@ -453,6 +464,10 @@ def _solve_single_freq(
 
         The fields are rotated from propagation coordinates back to global coordinates.
         """
+
+        if not LOCAL_SOLVER_IMPORTED:
+            raise ImportError(IMPORT_ERROR_MSG)
+
         solver_fields, n_complex, eps_spec = compute_modes(
             eps_cross=self._solver_eps(freq),
             coords=coords,
@@ -628,9 +643,10 @@ def _has_complex_eps(self) -> bool:
         eps and mu and uses a tolerance to determine whether to use real or complex fields, so
         the actual behavior may differ from what's predicted by this property."""
         for int_mat in self._intersecting_media:
-            if EigSolver.isinstance_complex(int_mat.eps_model(np.array(self.freqs))):
-                return True
-        return False
+            max_imag_eps = np.amax(np.abs(np.imag(int_mat.eps_model(np.array(self.freqs)))))
+            if not np.isclose(max_imag_eps, 0):
+                return False
+        return True
 
     def to_source(
         self,