Repository: pybamm-team/PyBaMM (python)

Issue:
OCP curves from data
**Summary**
It would be useful to define a function within PyBaMM that, given a data file, produces the interpolating function and also its Jacobian within `autograd`. That would be useful for OCPs, but shouldn't limit to that as some other parameter function might be provided as a dataset of experimental measurements (e.g. the entropic change).

I have already the code that produces this in the main script, but I think it would be a good idea to have it as a PyBaMM function that deals with it behind the scenes. 

 **Motivation**
 In many cases, the parameter functions come from experimental data so interpolation is a straightforward way to include them in the simulations. 

**Additional context**
The code in my main script looks like
```
data_cathode = pd.read_csv("my_data.csv")

interpolated_OCV_cathode = interpolate.CubicSpline(
    data_cathode.to_numpy()[:,0], 
    data_cathode.to_numpy()[:,1], 
    extrapolate=True
)

dOCV_cathode = interpolated_OCV_cathode.derivative()

@primitive
def OCV_cathode(sto):
    out = interpolated_OCV_cathode(sto)
    if np.size(out) == 1:
        out = np.array([out])[0]
    return out

def OCV_cathode_vjp(ans,sto):
    sto_shape = sto.shape
    return lambda g: np.full(sto_shape, g) * dOCV_cathode(sto)

defvjp(OCV_cathode, OCV_cathode_vjp)
```


Pull request description:
Issue 600 interpolate
# Description

Add `Interpolant` class to interpolate experimental data (e.g. OCP curves).

Fixes #600 

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)


# Key checklist:

- [ ] No style issues: `$ flake8`
- [ ] All tests pass: `$ python run-tests.py --unit`
- [ ] The documentation builds: `$ cd docs` and then `$ make clean; make html`

You can run all three at once, using `$ python run-tests.py --quick`.

## Further checks: 

- [ ] Code is commented, particularly in hard-to-understand areas
- [ ] Tests added that prove fix is effective or that feature works
- [ ] Any dependent changes have been merged and published in downstream modules


Patch:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index a47ab6a02..d5b88278b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Features
 
+- Add `Interpolant` class to interpolate experimental data (e.g. OCP curves) (#661)
 - Allow parameters to be set by material or by specifying a particular paper (#647)
 - Set relative and absolute tolerances independently in solvers (#645)
 
diff --git a/docs/source/expression_tree/index.rst b/docs/source/expression_tree/index.rst
index 8c2099ac2..2e95d07b1 100644
--- a/docs/source/expression_tree/index.rst
+++ b/docs/source/expression_tree/index.rst
@@ -17,4 +17,5 @@ Expression Tree
   broadcasts
   simplify
   functions
+  interpolant
   evaluate
diff --git a/docs/source/expression_tree/interpolant.rst b/docs/source/expression_tree/interpolant.rst
new file mode 100644
index 000000000..69ad86e02
--- /dev/null
+++ b/docs/source/expression_tree/interpolant.rst
@@ -0,0 +1,5 @@
+Interpolant
+===========
+
+.. autoclass:: pybamm.Interpolant
+  :members:
diff --git a/input/parameters/lithium-ion/cathodes/lico2_Marquis2019/lico2_data_example.csv b/input/parameters/lithium-ion/cathodes/lico2_Marquis2019/lico2_data_example.csv
new file mode 100644
index 000000000..f2f1809c7
--- /dev/null
+++ b/input/parameters/lithium-ion/cathodes/lico2_Marquis2019/lico2_data_example.csv
@@ -0,0 +1,50 @@
+0.000000000000000000e+00 4.714135898019971016e+00
+2.040816326530612082e-02 4.708899441575220557e+00
+4.081632653061224164e-02 4.702448345762175741e+00
+6.122448979591836593e-02 4.694558534379876136e+00
+8.163265306122448328e-02 4.684994372928071193e+00
+1.020408163265306006e-01 4.673523893805322516e+00
+1.224489795918367319e-01 4.659941254449398329e+00
+1.428571428571428492e-01 4.644096031712390271e+00
+1.632653061224489666e-01 4.625926611260677390e+00
+1.836734693877550839e-01 4.605491824833229053e+00
+2.040816326530612013e-01 4.582992038370575116e+00
+2.244897959183673186e-01 4.558769704421606228e+00
+2.448979591836734637e-01 4.533281647154224103e+00
+2.653061224489795533e-01 4.507041620859735254e+00
+2.857142857142856984e-01 4.480540404981123714e+00
+3.061224489795917880e-01 4.454158468368703439e+00
+3.265306122448979331e-01 4.428089899175588151e+00
+3.469387755102040782e-01 4.402295604083254155e+00
+3.673469387755101678e-01 4.376502631465185367e+00
+3.877551020408163129e-01 4.350272100879827519e+00
+4.081632653061224025e-01 4.323179536958428493e+00
+4.285714285714285476e-01 4.295195829713853719e+00
+4.489795918367346372e-01 4.267407675466301065e+00
+4.693877551020407823e-01 4.243081968022011985e+00
+4.897959183673469274e-01 4.220583168834260768e+00
+5.102040816326530726e-01 4.177032236370062712e+00
+5.306122448979591066e-01 4.134943568540559333e+00
+5.510204081632652517e-01 4.075402582839823928e+00
+5.714285714285713969e-01 4.055407164381796825e+00
+5.918367346938775420e-01 4.036052896449991323e+00
+6.122448979591835760e-01 4.012970397550268409e+00
+6.326530612244897211e-01 3.990385577539371287e+00
+6.530612244897958663e-01 3.970744780585252709e+00
+6.734693877551020114e-01 3.954753574690877738e+00
+6.938775510204081565e-01 3.942237451863396025e+00
+7.142857142857141906e-01 3.932683425747200534e+00
+7.346938775510203357e-01 3.925509771581312979e+00
+7.551020408163264808e-01 3.920182838859009422e+00
+7.755102040816326259e-01 3.916256861206461881e+00
+7.959183673469386600e-01 3.913378070528176877e+00
+8.163265306122448051e-01 3.911274218446639583e+00
+8.367346938775509502e-01 3.909739285381772067e+00
+8.571428571428570953e-01 3.908613829807601192e+00
+8.775510204081632404e-01 3.907726324580658162e+00
+8.979591836734692745e-01 3.906474088522892796e+00
+9.183673469387754196e-01 3.900204875423951556e+00
+9.387755102040815647e-01 3.848912814816038974e+00
+9.591836734693877098e-01 3.445226042113884724e+00
+9.795918367346938549e-01 1.687177743081021308e+00
+1.000000000000000000e+00 6.378908986260003328e-03
diff --git a/input/parameters/lithium-ion/cathodes/lico2_Marquis2019/lico2_ocp_Dualfoil1998.py b/input/parameters/lithium-ion/cathodes/lico2_Marquis2019/lico2_ocp_Dualfoil1998.py
index e63d8c707..1d6f3a306 100644
--- a/input/parameters/lithium-ion/cathodes/lico2_Marquis2019/lico2_ocp_Dualfoil1998.py
+++ b/input/parameters/lithium-ion/cathodes/lico2_Marquis2019/lico2_ocp_Dualfoil1998.py
@@ -3,23 +3,23 @@ import autograd.numpy as np
 
 def lico2_ocp_Dualfoil1998(sto):
     """
-        Lithium Cobalt Oxide (LiCO2) Open Circuit Potential (OCP) as a a function of the
-        stochiometry. The fit is taken from Dualfoil [1]. Dualfoil states that the data
-        was measured by Oscar Garcia 2001 using Quallion electrodes for 0.5 < sto < 0.99
-        and by Marc Doyle for sto<0.4 (for unstated electrodes). We could not find any
-        other records of the Garcia measurements. Doyles fits can be found in his
-        thesis [2] but we could not find any other record of his measurments.
-
-        References
-        ----------
-        .. [1] http://www.cchem.berkeley.edu/jsngrp/fortran.html
-        .. [2] CM Doyle. Design and simulation of lithium rechargeable batteries,
-               1995.
-
-          Parameters
-          ----------
-          sto: double
-               Stochiometry of material (li-fraction)
+    Lithium Cobalt Oxide (LiCO2) Open Circuit Potential (OCP) as a a function of the
+    stochiometry. The fit is taken from Dualfoil [1]. Dualfoil states that the data
+    was measured by Oscar Garcia 2001 using Quallion electrodes for 0.5 < sto < 0.99
+    and by Marc Doyle for sto<0.4 (for unstated electrodes). We could not find any
+    other records of the Garcia measurements. Doyles fits can be found in his
+    thesis [2] but we could not find any other record of his measurments.
+
+    References
+    ----------
+    .. [1] http://www.cchem.berkeley.edu/jsngrp/fortran.html
+    .. [2] CM Doyle. Design and simulation of lithium rechargeable batteries,
+           1995.
+
+    Parameters
+    ----------
+    sto: double
+       Stochiometry of material (li-fraction)
 
     """
 
diff --git a/pybamm/__init__.py b/pybamm/__init__.py
index 4f2f3680d..7723eb874 100644
--- a/pybamm/__init__.py
+++ b/pybamm/__init__.py
@@ -126,6 +126,7 @@ from .expression_tree.unary_operators import (
     r_average,
 )
 from .expression_tree.functions import *
+from .expression_tree.interpolant import Interpolant
 from .expression_tree.parameter import Parameter, FunctionParameter
 from .expression_tree.broadcasts import Broadcast, PrimaryBroadcast, FullBroadcast
 from .expression_tree.scalar import Scalar
diff --git a/pybamm/expression_tree/functions.py b/pybamm/expression_tree/functions.py
index 2bacdb792..20f6ae46e 100644
--- a/pybamm/expression_tree/functions.py
+++ b/pybamm/expression_tree/functions.py
@@ -18,20 +18,26 @@ class Function(pybamm.Symbol):
         func(child0.evaluate(t, y), child1.evaluate(t, y), etc).
     children : :class:`pybamm.Symbol`
         The children nodes to apply the function to
-
+    derivative : str, optional
+        Which derivative to use when differentiating ("autograd" or "derivative").
+        Default is "autograd".
     **Extends:** :class:`pybamm.Symbol`
     """
 
-    def __init__(self, function, *children):
+    def __init__(self, function, *children, name=None, derivative="autograd"):
 
-        try:
-            name = "function ({})".format(function.__name__)
-        except AttributeError:
-            name = "function ({})".format(function.__class__)
+        if name is not None:
+            self.name = name
+        else:
+            try:
+                name = "function ({})".format(function.__name__)
+            except AttributeError:
+                name = "function ({})".format(function.__class__)
         children_list = list(children)
         domain = self.get_children_domains(children_list)
 
         self.function = function
+        self.derivative = derivative
 
         # hack to work out whether function takes any params
         # (signature doesn't work for numpy)
@@ -73,7 +79,7 @@ class Function(pybamm.Symbol):
                 # if variable appears in the function,use autograd to differentiate
                 # function, and apply chain rule
                 if variable.id in [symbol.id for symbol in child.pre_order()]:
-                    partial_derivatives[i] = child.diff(variable) * self._diff(children)
+                    partial_derivatives[i] = self._diff(children) * child.diff(variable)
 
             # remove None entries
             partial_derivatives = list(filter(None, partial_derivatives))
@@ -86,7 +92,13 @@ class Function(pybamm.Symbol):
 
     def _diff(self, children):
         """ See :meth:`pybamm.Symbol._diff()`. """
-        return Function(autograd.elementwise_grad(self.function), *children)
+        if self.derivative == "autograd":
+            return Function(autograd.elementwise_grad(self.function), *children)
+        elif self.derivative == "derivative":
+            # keep using "derivative" as derivative
+            return pybamm.Function(
+                self.function.derivative(), *children, derivative="derivative"
+            )
 
     def _jac(self, variable):
         """ See :meth:`pybamm.Symbol._jac()`. """
@@ -158,7 +170,9 @@ class Function(pybamm.Symbol):
             : :pybamm.Function
             A new copy of the function
         """
-        return pybamm.Function(self.function, *children)
+        return pybamm.Function(
+            self.function, *children, name=self.name, derivative=self.derivative
+        )
 
     def _function_simplify(self, simplified_children):
         """
@@ -181,7 +195,12 @@ class Function(pybamm.Symbol):
             # If self.function() is a constant current then simplify to scalar
             return pybamm.Scalar(self.function.parameters_eval["Current [A]"])
         else:
-            return pybamm.Function(self.function, *simplified_children)
+            return pybamm.Function(
+                self.function,
+                *simplified_children,
+                name=self.name,
+                derivative=self.derivative
+            )
 
 
 class SpecificFunction(Function):
@@ -233,7 +252,7 @@ class Cosh(SpecificFunction):
         super().__init__(np.cosh, child)
 
     def _diff(self, children):
-        """ See :meth:`pybamm.Symbol._diff()`. """
+        """ See :meth:`pybamm.Function._diff()`. """
         return Sinh(children[0])
 
 
@@ -249,7 +268,7 @@ class Exponential(SpecificFunction):
         super().__init__(np.exp, child)
 
     def _diff(self, children):
-        """ See :meth:`pybamm.Symbol._diff()`. """
+        """ See :meth:`pybamm.Function._diff()`. """
         return Exponential(children[0])
 
 
@@ -265,7 +284,7 @@ class Log(SpecificFunction):
         super().__init__(np.log, child)
 
     def _diff(self, children):
-        """ See :meth:`pybamm.Symbol._diff()`. """
+        """ See :meth:`pybamm.Function._diff()`. """
         return 1 / children[0]
 
 
@@ -291,7 +310,7 @@ class Sin(SpecificFunction):
         super().__init__(np.sin, child)
 
     def _diff(self, children):
-        """ See :meth:`pybamm.Symbol._diff()`. """
+        """ See :meth:`pybamm.Function._diff()`. """
         return Cos(children[0])
 
 
@@ -307,7 +326,7 @@ class Sinh(SpecificFunction):
         super().__init__(np.sinh, child)
 
     def _diff(self, children):
-        """ See :meth:`pybamm.Symbol._diff()`. """
+        """ See :meth:`pybamm.Function._diff()`. """
         return Cosh(children[0])
 
 
diff --git a/pybamm/expression_tree/interpolant.py b/pybamm/expression_tree/interpolant.py
new file mode 100644
index 000000000..5cb8a9c6c
--- /dev/null
+++ b/pybamm/expression_tree/interpolant.py
@@ -0,0 +1,64 @@
+#
+# Interpolating class
+#
+import pybamm
+from scipy import interpolate
+
+
+class Interpolant(pybamm.Function):
+    """
+    Interpolate data in 1D.
+
+    Parameters
+    ----------
+    data : :class:`numpy.ndarray`
+        Numpy array of data to use for interpolation. Must have exactly two columns (x
+        and y data)
+    child : :class:`pybamm.Symbol`
+        Node to use when evaluating the interpolant
+    name : str, optional
+        Name of the interpolant. Default is None, in which case the name "interpolating
+        function" is given.
+    interpolator : str, optional
+        Which interpolator to use ("pchip" or "cubic spline"). Note that whichever
+        interpolator is used must be differentiable (for ``Interpolator._diff``).
+        Default is "cubic spline". Note that "pchip" may give slow results.
+    extrapolate : bool, optional
+        Whether to extrapolate for points that are outside of the parametrisation
+        range, or return NaN (following default behaviour from scipy). Default is True.
+
+    **Extends**: :class:`pybamm.Function`
+    """
+
+    def __init__(
+        self, data, child, name=None, interpolator="cubic spline", extrapolate=True
+    ):
+        if data.ndim != 2 or data.shape[1] != 2:
+            raise ValueError(
+                """
+                data should have exactly two columns (x and y) but has shape {}
+                """.format(
+                    data.shape
+                )
+            )
+        elif interpolator == "pchip":
+            interpolating_function = interpolate.PchipInterpolator(
+                data[:, 0], data[:, 1], extrapolate=extrapolate
+            )
+        elif interpolator == "cubic spline":
+            interpolating_function = interpolate.CubicSpline(
+                data[:, 0], data[:, 1], extrapolate=extrapolate
+            )
+        else:
+            raise ValueError("interpolator '{}' not recognised".format(interpolator))
+        # Set name
+        if name is not None:
+            name = "interpolating function ({})".format(name)
+        else:
+            name = "interpolating function"
+        super().__init__(
+            interpolating_function, child, name=name, derivative="derivative"
+        )
+        # Store information as attributes
+        self.interpolator = interpolator
+        self.extrapolate = extrapolate
diff --git a/pybamm/expression_tree/simplify.py b/pybamm/expression_tree/simplify.py
index 97ac0ab59..7df95240c 100644
--- a/pybamm/expression_tree/simplify.py
+++ b/pybamm/expression_tree/simplify.py
@@ -16,7 +16,9 @@ def simplify_if_constant(symbol):
     if symbol.is_constant():
         result = symbol.evaluate_ignoring_errors()
         if result is not None:
-            if isinstance(result, numbers.Number):
+            if isinstance(result, numbers.Number) or (
+                isinstance(result, np.ndarray) and result.ndim == 0
+            ):
                 return pybamm.Scalar(result)
             elif isinstance(result, np.ndarray) or issparse(result):
                 if result.ndim == 1 or result.shape[1] == 1:
diff --git a/pybamm/parameters/parameter_values.py b/pybamm/parameters/parameter_values.py
index cb2fec319..9473f481c 100644
--- a/pybamm/parameters/parameter_values.py
+++ b/pybamm/parameters/parameter_values.py
@@ -4,6 +4,7 @@
 import pybamm
 import pandas as pd
 import os
+import numpy as np
 
 
 class ParameterValues(dict):
@@ -101,26 +102,7 @@ class ParameterValues(dict):
                 os.path.join(component_path, "parameters.csv")
             )
             # Update parameters, making sure to check any conflicts
-            self.update(component_params, check_conflict=True)
-            # Load functions if they are specified
-            for name, param in component_params.items():
-                # Functions are flagged with the string "[function]"
-                if isinstance(param, str):
-                    if param.startswith("[function]"):
-                        self[name] = pybamm.load_function(
-                            os.path.join(component_path, param[10:] + ".py")
-                        )
-                    # Inbuilt functions are flagged with the string "[inbuilt]"
-                    elif param.startswith("[inbuilt class]"):
-                        # Extra set of brackets at the end makes an instance of the
-                        # class
-                        self[name] = getattr(pybamm, param[15:])()
-                    # Data is flagged with the string "[data]"
-                    # elif param.startswith("[data]"):
-                    # TODO: implement interpolating function for data
-                    # Anything else should be a converted to a float
-                    else:
-                        self[name] = float(param)
+            self.update(component_params, check_conflict=True, path=component_path)
 
     def read_parameters_csv(self, filename):
         """Reads parameters from csv file into dict.
@@ -141,23 +123,45 @@ class ParameterValues(dict):
         df.dropna(how="all", inplace=True)
         return {k: v for (k, v) in zip(df["Name [units]"], df["Value"])}
 
-    def update(self, values, check_conflict=False):
+    def update(self, values, check_conflict=False, path=""):
         # check parameter values
         values = self.check_and_update_parameter_values(values)
         # update
-        for k, v in values.items():
+        for name, value in values.items():
             # check for conflicts
             if (
                 check_conflict is True
-                and k in self.keys()
-                and not (self[k] == float(v) or self[k] == v)
+                and name in self.keys()
+                and not (self[name] == float(value) or self[name] == value)
             ):
                 raise ValueError(
-                    "parameter '{}' already defined with value '{}'".format(k, self[k])
+                    "parameter '{}' already defined with value '{}'".format(
+                        name, self[name]
+                    )
                 )
-            # if no conflicts, update
+            # if no conflicts, update, loading functions and data if they are specified
             else:
-                self[k] = v
+                # Functions are flagged with the string "[function]"
+                if isinstance(value, str):
+                    if value.startswith("[function]"):
+                        self[name] = pybamm.load_function(
+                            os.path.join(path, value[10:] + ".py")
+                        )
+                    # Inbuilt functions are flagged with the string "[inbuilt]"
+                    elif value.startswith("[inbuilt class]"):
+                        # Extra set of brackets at the end makes an instance of the
+                        # class
+                        self[name] = getattr(pybamm, value[15:])()
+                    # Data is flagged with the string "[data]"
+                    elif value.startswith("[data]"):
+                        data = np.loadtxt(os.path.join(path, value[6:] + ".csv"))
+                        # Save name and data
+                        self[name] = (value[6:], data)
+                    # Anything else should be a converted to a float
+                    else:
+                        self[name] = float(value)
+                else:
+                    self[name] = value
         # reset processed symbols
         self._processed_symbols = {}
 
@@ -392,8 +396,16 @@ class ParameterValues(dict):
                 if isinstance(function_name, pybamm.GetCurrentData):
                     function_name.interpolate()
 
-            # Create Function object and differentiate if necessary
-            function = pybamm.Function(function_name, *new_children)
+            # Create Function or Interpolant objec
+            if isinstance(function_name, tuple):
+                # If function_name is a tuple then it should be (name, data) and we need
+                # to create an Interpolant
+                name, data = function_name
+                function = pybamm.Interpolant(data, *new_children, name=name)
+            else:
+                # otherwise create standard function
+                function = pybamm.Function(function_name, *new_children)
+            # Differentiate if necessary
             if symbol.diff_variable is None:
                 return function
             else:


Test patch:
diff --git a/tests/unit/test_expression_tree/test_interpolant.py b/tests/unit/test_expression_tree/test_interpolant.py
new file mode 100644
index 000000000..84de9eef7
--- /dev/null
+++ b/tests/unit/test_expression_tree/test_interpolant.py
@@ -0,0 +1,83 @@
+#
+# Tests for the Function classes
+#
+import pybamm
+
+import unittest
+import numpy as np
+
+
+class TestInterpolant(unittest.TestCase):
+    def test_errors(self):
+        with self.assertRaisesRegex(ValueError, "data should have exactly two columns"):
+            pybamm.Interpolant(np.ones(10), None)
+        with self.assertRaisesRegex(ValueError, "interpolator 'bla' not recognised"):
+            pybamm.Interpolant(np.ones((10, 2)), None, interpolator="bla")
+
+    def test_interpolation(self):
+        x = np.linspace(0, 1)[:, np.newaxis]
+        y = pybamm.StateVector(slice(0, 2))
+        # linear
+        linear = np.hstack([x, 2 * x])
+        for interpolator in ["pchip", "cubic spline"]:
+            interp = pybamm.Interpolant(linear, y, interpolator=interpolator)
+            np.testing.assert_array_almost_equal(
+                interp.evaluate(y=np.array([0.397, 1.5]))[:, 0], np.array([0.794, 3])
+            )
+        # square
+        square = np.hstack([x, x ** 2])
+        y = pybamm.StateVector(slice(0, 1))
+        for interpolator in ["pchip", "cubic spline"]:
+            interp = pybamm.Interpolant(square, y, interpolator=interpolator)
+            np.testing.assert_array_almost_equal(
+                interp.evaluate(y=np.array([0.397]))[:, 0], np.array([0.397 ** 2])
+            )
+
+        # with extrapolation set to False
+        for interpolator in ["pchip", "cubic spline"]:
+            interp = pybamm.Interpolant(
+                square, y, interpolator=interpolator, extrapolate=False
+            )
+            np.testing.assert_array_equal(
+                interp.evaluate(y=np.array([2]))[:, 0], np.array([np.nan])
+            )
+
+    def test_name(self):
+        a = pybamm.Symbol("a")
+        x = np.linspace(0, 1)[:, np.newaxis]
+        interp = pybamm.Interpolant(np.hstack([x, x]), a, "name")
+        self.assertEqual(interp.name, "interpolating function (name)")
+
+    def test_diff(self):
+        x = np.linspace(0, 1)[:, np.newaxis]
+        y = pybamm.StateVector(slice(0, 2))
+        # linear (derivative should be 2)
+        linear = np.hstack([x, 2 * x])
+        for interpolator in ["pchip", "cubic spline"]:
+            interp_diff = pybamm.Interpolant(linear, y, interpolator=interpolator).diff(
+                y
+            )
+            np.testing.assert_array_almost_equal(
+                interp_diff.evaluate(y=np.array([0.397, 1.5]))[:, 0], np.array([2, 2])
+            )
+        # square (derivative should be 2*x)
+        square = np.hstack([x, x ** 2])
+        for interpolator in ["pchip", "cubic spline"]:
+            interp_diff = pybamm.Interpolant(square, y, interpolator=interpolator).diff(
+                y
+            )
+            np.testing.assert_array_almost_equal(
+                interp_diff.evaluate(y=np.array([0.397, 0.806]))[:, 0],
+                np.array([0.794, 1.612]),
+                decimal=3,
+            )
+
+
+if __name__ == "__main__":
+    print("Add -v for more debug output")
+    import sys
+
+    if "-v" in sys.argv:
+        debug = True
+    pybamm.settings.debug_mode = True
+    unittest.main()
diff --git a/tests/unit/test_parameters/test_parameter_values.py b/tests/unit/test_parameters/test_parameter_values.py
index 08311bcd6..db92388f9 100644
--- a/tests/unit/test_parameters/test_parameter_values.py
+++ b/tests/unit/test_parameters/test_parameter_values.py
@@ -2,10 +2,10 @@
 # Tests for the Base Parameter Values class
 #
 import pybamm
-
-import unittest
+import os
 import numpy as np
 
+import unittest
 import tests.shared as shared
 
 
@@ -25,7 +25,7 @@ class TestParameterValues(unittest.TestCase):
             values="input/parameters/lithium-ion/cathodes/lico2_Marquis2019/"
             + "parameters.csv"
         )
-        self.assertEqual(param["Reference temperature [K]"], "298.15")
+        self.assertEqual(param["Reference temperature [K]"], 298.15)
 
         # values vs chemistry
         with self.assertRaisesRegex(
@@ -294,6 +294,61 @@ class TestParameterValues(unittest.TestCase):
         self.assertIsInstance(processed_func, pybamm.Function)
         self.assertEqual(processed_func.evaluate(), 3)
 
+    def test_process_interpolant(self):
+        x = np.linspace(0, 10)[:, np.newaxis]
+        data = np.hstack([x, 2 * x])
+        parameter_values = pybamm.ParameterValues(
+            {"a": 3.01, "Diffusivity": ("times two", data)}
+        )
+
+        a = pybamm.Parameter("a")
+        func = pybamm.FunctionParameter("Diffusivity", a)
+
+        processed_func = parameter_values.process_symbol(func)
+        self.assertIsInstance(processed_func, pybamm.Interpolant)
+        self.assertEqual(processed_func.evaluate(), 6.02)
+
+        # process differentiated function parameter
+        diff_func = func.diff(a)
+        processed_diff_func = parameter_values.process_symbol(diff_func)
+        self.assertEqual(processed_diff_func.evaluate(), 2)
+
+    def test_interpolant_against_function(self):
+        parameter_values = pybamm.ParameterValues({"a": 0.6})
+        parameter_values.update(
+            {
+                "function": "[function]lico2_ocp_Dualfoil1998",
+                "interpolation": "[data]lico2_data_example",
+            },
+            path=os.path.join(
+                pybamm.root_dir(),
+                "input",
+                "parameters",
+                "lithium-ion",
+                "cathodes",
+                "lico2_Marquis2019",
+            ),
+        )
+
+        a = pybamm.Parameter("a")
+        func = pybamm.FunctionParameter("function", a)
+        interp = pybamm.FunctionParameter("interpolation", a)
+
+        processed_func = parameter_values.process_symbol(func)
+        processed_interp = parameter_values.process_symbol(interp)
+        np.testing.assert_array_almost_equal(
+            processed_func.evaluate(), processed_interp.evaluate(), decimal=4
+        )
+
+        # process differentiated function parameter
+        diff_func = func.diff(a)
+        diff_interp = interp.diff(a)
+        processed_diff_func = parameter_values.process_symbol(diff_func)
+        processed_diff_interp = parameter_values.process_symbol(diff_interp)
+        np.testing.assert_array_almost_equal(
+            processed_diff_func.evaluate(), processed_diff_interp.evaluate(), decimal=2
+        )
+
     def test_process_complex_expression(self):
         var1 = pybamm.Variable("var1")
         var2 = pybamm.Variable("var2")