include helper function to get positions dictionaries

Author: reint-fischer

src/parcels/_core/field.py

@@ -216,12 +216,7 @@ def eval(self, time: datetime, z, y, x, particles=None, applyConversion=True):
         else:
             _ei = particles.ei[:, self.igrid]
 
-        particle_positions = {"time": time, "z": z, "lat": y, "lon": x}
-        grid_positions = {}
-        grid_positions.update(_search_time_index(self, time))
-        grid_positions.update(self.grid.search(z, y, x, ei=_ei))
-        _update_particles_ei(particles, grid_positions, self)
-        _update_particle_states_position(particles, grid_positions)
+        particle_positions, grid_positions = _get_positions(self, time, z, y, x, particles, _ei)
 
         value = self._interp_method(particle_positions, grid_positions, self)
 
@@ -302,12 +297,7 @@ def eval(self, time: datetime, z, y, x, particles=None, applyConversion=True):
         else:
             _ei = particles.ei[:, self.igrid]
 
-        particle_positions = {"time": time, "z": z, "lat": y, "lon": x}
-        grid_positions = {}
-        grid_positions.update(_search_time_index(self.U, time))
-        grid_positions.update(self.grid.search(z, y, x, ei=_ei))
-        _update_particles_ei(particles, grid_positions, self)
-        _update_particle_states_position(particles, grid_positions)
+        particle_positions, grid_positions = _get_positions(self.U, time, z, y, x, particles, _ei)
 
         if self._vector_interp_method is None:
             u = self.U._interp_method(particle_positions, grid_positions, self.U)
@@ -343,7 +333,7 @@ def __getitem__(self, key):
             return _deal_with_errors(error, key, vector_type=self.vector_type)
 
 
-def _update_particles_ei(particles, grid_positions, field):
+def _update_particles_ei(particles, grid_positions: dict, field: Field):
     """Update the element index (ei) of the particles"""
     if particles is not None:
         if isinstance(field.grid, XGrid):
@@ -363,7 +353,7 @@ def _update_particles_ei(particles, grid_positions, field):
             )
 
 
-def _update_particle_states_position(particles, grid_positions):
+def _update_particle_states_position(particles, grid_positions: dict):
     """Update the particle states based on the position dictionary."""
     if particles:  # TODO also support uxgrid search
         for dim in ["X", "Y"]:
@@ -478,3 +468,14 @@ def _assert_same_time_interval(fields: list[Field]) -> None:
             raise ValueError(
                 f"Fields must have the same time domain. {fields[0].name}: {reference_time_interval}, {field.name}: {field.time_interval}"
             )
+
+
+def _get_positions(field: Field, time, z, y, x, particles, _ei) -> tuple[dict, dict]:
+    """Initialize and populate particle_positions and grid_positions dictionaries"""
+    particle_positions = {"time": time, "z": z, "lat": y, "lon": x}
+    grid_positions = {}
+    grid_positions.update(_search_time_index(field, time))
+    grid_positions.update(field.grid.search(z, y, x, ei=_ei))
+    _update_particles_ei(particles, grid_positions, field)
+    _update_particle_states_position(particles, grid_positions)
+    return particle_positions, grid_positions