Implemented non_linear_time, an array of time as an input to step function

CHANGELOG.md

@@ -6,6 +6,7 @@
 - Renamed "electrode diffusivity" to "particle diffusivity" as a non-breaking change with a deprecation warning ([#3624](https://github.com/pybamm-team/PyBaMM/pull/3624))
 - Add support for BPX version 0.4.0 which allows for blended electrodes and user-defined parameters in BPX([#3414](https://github.com/pybamm-team/PyBaMM/pull/3414))
 - Added the ability to specify a custom solver tolerance in `get_initial_stoichiometries` and related functions ([#3714](https://github.com/pybamm-team/PyBaMM/pull/3714))
+- Modified `step` function to take an array of time `t_eval` as an argument and deprecated use of `npts`. ([#3627](https://github.com/pybamm-team/PyBaMM/pull/3627))
 
 ## Bug Fixes
 

examples/scripts/SPMe_step.py

@@ -32,12 +32,14 @@
 
 # step model
 dt = 500
+# t_eval is an array of time in the interval 0 to dt, dt being size of the step.
+t_eval = np.array([0, 50, 100, 200, 500])
 time = 0
 end_time = solution.t[-1]
 step_solver = pybamm.CasadiSolver()
 step_solution = None
 while time < end_time:
-    step_solution = step_solver.step(step_solution, model, dt=dt, npts=10)
+    step_solution = step_solver.step(step_solution, model, dt=dt, t_eval=t_eval)
     time += dt
 
 # plot

pybamm/simulation.py

@@ -786,7 +786,7 @@ def solve(
                             current_solution,
                             model,
                             dt,
-                            npts=npts,
+                            t_eval=np.linspace(0, dt, npts),
                             save=False,
                             **kwargs,
                         )
@@ -967,15 +967,21 @@ def run_padding_rest(self, kwargs, rest_time, step_solution):
             step_solution,
             model,
             rest_time,
-            npts=npts,
+            t_eval=np.linspace(0, rest_time, npts),
             save=False,
             **kwargs,
         )
 
         return step_solution_with_rest
 
     def step(
-        self, dt, solver=None, npts=2, save=True, starting_solution=None, **kwargs
+        self,
+        dt,
+        solver=None,
+        t_eval=None,
+        save=True,
+        starting_solution=None,
+        **kwargs,
     ):
         """
         A method to step the model forward one timestep. This method will
@@ -987,9 +993,10 @@ def step(
             The timestep over which to step the solution
         solver : :class:`pybamm.BaseSolver`
             The solver to use to solve the model.
-        npts : int, optional
-            The number of points at which the solution will be returned during
-            the step dt. Default is 2 (returns the solution at t0 and t0 + dt).
+        t_eval : list or numpy.ndarray, optional
+            An array of times at which to return the solution during the step
+            (Note: t_eval is the time measured from the start of the step, so should start at 0 and end at dt).
+            By default, the solution is returned at t0 and t0 + dt.
         save : bool
             Turn on to store the solution of all previous timesteps
         starting_solution : :class:`pybamm.Solution`
@@ -1009,7 +1016,12 @@ def step(
             starting_solution = self._solution
 
         self._solution = solver.step(
-            starting_solution, self.built_model, dt, npts=npts, save=save, **kwargs
+            starting_solution,
+            self.built_model,
+            dt,
+            t_eval=t_eval,
+            save=save,
+            **kwargs,
         )
 
         return self.solution

pybamm/solvers/base_solver.py

@@ -1102,7 +1102,8 @@ def step(
         old_solution,
         model,
         dt,
-        npts=2,
+        t_eval=None,
+        npts=None,
         inputs=None,
         save=True,
     ):
@@ -1120,9 +1121,11 @@ def step(
             initial_conditions
         dt : numeric type
             The timestep (in seconds) over which to step the solution
-        npts : int, optional
-            The number of points at which the solution will be returned during
-            the step dt. default is 2 (returns the solution at t0 and t0 + dt).
+        t_eval : list or numpy.ndarray, optional
+            An array of times at which to return the solution during the step
+            (Note: t_eval is the time measured from the start of the step, so should start at 0 and end at dt).
+            By default, the solution is returned at t0 and t0 + dt.
+        npts : deprecated
         inputs : dict, optional
             Any input parameters to pass to the model when solving
         save : bool
@@ -1161,10 +1164,28 @@ def step(
                 f"Step time must be at least {pybamm.TimerTime(step_start_offset)}"
             )
 
+        # Raise deprecation warning for npts and convert it to t_eval
+        if npts is not None:
+            warnings.warn(
+                "The 'npts' parameter is deprecated, use 't_eval' instead.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+            t_eval = np.linspace(0, dt, npts)
+
+        if t_eval is not None:
+            # Checking if t_eval lies within range
+            if t_eval[0] != 0 or t_eval[-1] != dt:
+                raise pybamm.SolverError(
+                    "Elements inside array t_eval must lie in the closed interval 0 to dt"
+                )
+
+        else:
+            t_eval = np.array([0, dt])
+
         t_start = old_solution.t[-1]
+        t_eval = t_start + t_eval
         t_end = t_start + dt
-        # Calculate t_eval
-        t_eval = np.linspace(t_start, t_end, npts)
 
         if t_start == 0:
             t_start_shifted = t_start
@@ -1248,15 +1269,9 @@ def step(
         # Report times
         pybamm.logger.verbose(f"Finish stepping {model.name} ({termination})")
         pybamm.logger.verbose(
-            (
-                "Set-up time: {}, Step time: {} (of which integration time: {}), "
-                "Total time: {}"
-            ).format(
-                solution.set_up_time,
-                solution.solve_time,
-                solution.integration_time,
-                solution.total_time,
-            )
+            f"Set-up time: {solution.set_up_time}, Step time: {solution.solve_time}"
+            f"(of which integration time: {solution.integration_time}), "
+            f"Total time: {solution.total_time}"
         )
 
         # Return solution

tests/unit/test_solvers/test_base_solver.py

@@ -75,6 +75,22 @@ def test_nonmonotonic_teval(self):
         ):
             solver.step(None, model, dt)
 
+        # Checking if array t_eval lies within range
+        dt = 2
+        t_eval = np.array([0, 1])
+        with self.assertRaisesRegex(
+            pybamm.SolverError,
+            "Elements inside array t_eval must lie in the closed interval 0 to dt",
+        ):
+            solver.step(None, model, dt, t_eval=t_eval)
+
+        t_eval = np.array([1, dt])
+        with self.assertRaisesRegex(
+            pybamm.SolverError,
+            "Elements inside array t_eval must lie in the closed interval 0 to dt",
+        ):
+            solver.step(None, model, dt, t_eval=t_eval)
+
     def test_solution_time_length_fail(self):
         model = pybamm.BaseModel()
         v = pybamm.Scalar(1)