[Bug]: Conversion to scalars for arrays with ndim>0: NumPy 1.25 deprecation (Python 3.9–3.11)

[agriyakhetarpal]

PyBaMM Version

23.5

Python Version

3.11

Describe the bug

From NumPy 1.25 Release Notes – Deprecations

Only ndim-0 arrays are treated as scalars. NumPy used to treat all arrays of size 1 (e.g., np.array([3.14])) as scalars. In the future, this will be limited to arrays of ndim 0 (e.g., np.array(3.14)). The following expressions will report a deprecation warning:

a = np.array([3.14])
float(a)  # better: a[0] to get the numpy.float or a.item()

b = np.array([[3.14]])
c = numpy.random.rand(10)
c[0] = b  # better: c[0] = b[0, 0]

in numpy/numpy#10404 and numpy/numpy#10615

We convert arrays to scalars with float() in six locations, which bring up warnings as shown below:

Steps to Reproduce

Run the test suite

tox -e unit

Relevant log output

/Users/agriyakhetarpal/Desktop/PyBaMM/pybamm/models/full_battery_models/lithium_ion/electrode_soh.py:428: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)

PyBaMM/pybamm/models/full_battery_models/lithium_ion/electrode_soh.py:625: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)

test_error (test_models.test_full_battery_models.test_lithium_ion.test_electrode_soh.TestElectrodeSOH.test_error) ... /Users/agriyakhetarpal/Desktop/PyBaMM/pybamm/models/full_battery_models/lithium_ion/electrode_soh.py:425: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
  V_lower_bound = float(

test_parameter_values_with_model (test_parameters.test_parameter_sets.test_parameters_with_default_models.TestParameterValuesWithModel.test_parameter_values_with_model) ... /Users/agriyakhetarpal/Desktop/PyBaMM/pybamm/parameters/parameter_values.py:650: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)

test_convergence_without_bcs (test_spatial_methods.test_finite_volume.test_extrapolation.TestExtrapolation.test_convergence_without_bcs) ... /Users/agriyakhetarpal/Desktop/PyBaMM/tests/unit/test_spatial_methods/test_finite_volume/test_extrapolation.py:54: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)

test_model_solver_events (test_solvers.test_casadi_solver.TestCasadiSolver.test_model_solver_events) ... /Users/agriyakhetarpal/Desktop/PyBaMM/pybamm/solvers/base_solver.py:505: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)

[agriyakhetarpal]

I do think I am filing this issue a bit early since we still support Python 3.8 (equivalent to NumPy 1.24.X), while these warnings only occur on Python 3.9–3.11 (NumPy 1.25)

Nonetheless, I managed to fix this (partially), I shall open a PR soon