[Bug]: IDAKLU output_variables fails when checking for interpolant_extrapolation events

[pipliggins]

PyBaMM Version

develop

Python Version

3.11.6

Describe the bug

When running a model with the IDAKLU solver and some output_variables and the solution contains INTERPOLANT_EXTRAPOLATION events, pybamm complains about y_slice vectors being too short.

The code to reproduce is taken from #4414 .

Steps to Reproduce

import pybamm
import numpy as np

# Define inputs
inputs = {
    "Current function [A]": 0.222,
    "Separator porosity": 0.3,
}

# Set-up the model
options = {"cell geometry": "arbitrary", "thermal": "lumped"}
model = pybamm.lithium_ion.DFN(options=options)
geometry = model.default_geometry
# param = model.default_parameter_values
param = pybamm.ParameterValues.create_from_bpx(r"lfp_18650_cell_BPX.json")
param.update(inputs)
var = pybamm.standard_spatial_vars
var_pts = {var.x_n: 20, var.x_s: 20, var.x_p: 20, var.r_n: 10, var.r_p: 10}

# Use a short time-vector for this example
t_eval = np.linspace(0, 360, 10)

# Include the variable of interest in output_variables
output_variables = [
    "Voltage [V]",
    "Current [A]",
    "Time [min]",
    "X-averaged cell temperature [C]",  # want to retrieve this and print the output
]

# Create the IDAKLU Solver object
idaklu_solver = pybamm.IDAKLUSolver(
    rtol=1e-6, atol=1e-6, output_variables=output_variables
)

sim = pybamm.Simulation(
    model,
    parameter_values=param,
    geometry=geometry,
    var_pts=var_pts,
    solver=idaklu_solver,
    output_variables=output_variables,
)
sol = sim.solve(
    t_eval=t_eval,
    calculate_sensitivities=True,
)

Relevant log output

File "/Users/pipliggins/Documents/repos/pybamm-local/issue_test_copy.py", line 47, in <module>
    sol = sim.solve(
          ^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/simulation.py", line 534, in solve
    self._solution = solver.solve(
                     ^^^^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/solvers/base_solver.py", line 957, in solve
    self.check_extrapolation(solution, model.events)
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/solvers/base_solver.py", line 1497, in check_extrapolation
    and event.expression.evaluate(t, y, inputs=inputs) < self.extrap_tol
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/expression_tree/functions.py", line 130, in evaluate
    evaluated_children = [
                         ^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/expression_tree/functions.py", line 131, in <listcomp>
    child.evaluate(t, y, y_dot, inputs) for child in self.children
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/expression_tree/binary_operators.py", line 160, in evaluate
    left = self.left.evaluate(t, y, y_dot, inputs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/expression_tree/binary_operators.py", line 160, in evaluate
    left = self.left.evaluate(t, y, y_dot, inputs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/expression_tree/binary_operators.py", line 161, in evaluate
    right = self.right.evaluate(t, y, y_dot, inputs)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/expression_tree/symbol.py", line 817, in evaluate
    return self._base_evaluate(t, y, y_dot, inputs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/pipliggins/Documents/repos/pybamm-local/src/pybamm/expression_tree/state_vector.py", line 292, in _base_evaluate
    raise ValueError(
ValueError: y is too short, so value with slice is smaller than expected

[BradyPlanden]

Just as an additional datapoint, we are seeing this on PyBOP as well. Thanks for the fix @pipliggins!