Adding support for subsecond dt values in pset.execute

parcels/particleset.py

@@ -425,6 +425,19 @@ def _num_error_particles(self):
         """
         return np.sum(np.isin(self._data["state"], [StatusCode.Success, StatusCode.Evaluate], invert=True))
 
+    def update_dt_dtype(self, dt_dtype: np.dtype):
+        """Update the dtype of dt
+
+        Parameters
+        ----------
+        dt_dtype : np.dtype
+            New dtype for dt.
+        """
+        if dt_dtype not in [np.timedelta64, "timedelta64[ns]", "timedelta64[ms]", "timedelta64[s]"]:
+            raise ValueError(f"dt_dtype must be a numpy timedelta64 dtype. Got {dt_dtype=!r}")
+
+        self._data["dt"] = self._data["dt"].astype(dt_dtype)
+
     def set_variable_write_status(self, var, write_status):
         """Method to set the write status of a Variable.
 
@@ -500,6 +513,17 @@ def execute(
         except (ValueError, AssertionError) as e:
             raise ValueError(f"dt must be a non-zero datetime.timedelta or np.timedelta64 object, got {dt=!r}") from e
 
+        # Check if particle dt has finer resolution than input dt
+        particle_resolution = np.timedelta64(1, np.datetime_data(self._data["dt"].dtype))
+        input_resolution = np.timedelta64(1, np.datetime_data(dt.dtype))
+
+        if input_resolution >= particle_resolution:
+            self._data["dt"][:] = dt
+        else:
+            raise ValueError(
+                f"The dtype of dt ({dt.dtype}) is coarser than the dtype of the particle dt ({self._data['dt'].dtype}). Please use ParticleSet.set_dt_dtype() to provide a dt with at least the same precision as the particle dt."
+            )
+
         if runtime is not None:
             try:
                 runtime = maybe_convert_python_timedelta_to_numpy(runtime)
@@ -508,8 +532,6 @@ def execute(
                     f"The runtime must be a datetime.timedelta or np.timedelta64 object. Got {type(runtime)}"
                 ) from e
 
-        self._data["dt"][:] = dt
-
         start_time, end_time = _get_simulation_start_and_end_times(
             self.fieldset.time_interval, self._data["time_nextloop"], runtime, endtime, sign_dt
         )

tests/v4/test_particleset_execute.py

@@ -12,6 +12,7 @@
     ParticleSet,
     StatusCode,
     UXPiecewiseConstantFace,
+    Variable,
     VectorField,
 )
 from parcels._datasets.structured.generated import simple_UV_dataset
@@ -150,6 +151,27 @@ def test_particleset_endtime_type(fieldset, endtime, expectation):
         pset.execute(endtime=endtime, dt=np.timedelta64(10, "m"), pyfunc=DoNothing)
 
 
+@pytest.mark.parametrize(
+    "dt", [np.timedelta64(1, "s"), np.timedelta64(1, "ms"), np.timedelta64(10, "ms"), np.timedelta64(1, "ns")]
+)
+def test_pset_execute_subsecond_dt(fieldset, dt):
+    def AddDt(particles, fieldset):  # pragma: no cover
+        dt = particles.dt / np.timedelta64(1, "s")
+        particles.added_dt += dt
+
+    pclass = Particle.add_variable(Variable("added_dt", dtype=np.float32, initial=0))
+    pset = ParticleSet(fieldset, pclass=pclass, lon=0, lat=0)
+    pset.update_dt_dtype(dt.dtype)
+    pset.execute(AddDt, runtime=dt * 10, dt=dt)
+    np.testing.assert_allclose(pset[0].added_dt, 10.0 * dt / np.timedelta64(1, "s"), atol=1e-5)
+
+
+def test_pset_execute_subsecond_dt_error(fieldset):
+    pset = ParticleSet(fieldset, lon=0, lat=0)
+    with pytest.raises(ValueError, match="The dtype of dt"):
+        pset.execute(DoNothing, runtime=np.timedelta64(10, "ms"), dt=np.timedelta64(1, "ms"))
+
+
 def test_pset_remove_particle_in_kernel(fieldset):
     npart = 100
     pset = ParticleSet(fieldset, lon=np.linspace(0, 1, npart), lat=np.linspace(1, 0, npart))