allow evaluation of an interpolant with a float

pybamm/expression_tree/interpolant.py

@@ -5,7 +5,7 @@
 import numpy as np
 from scipy import interpolate
 from typing import Sequence
-
+import numbers
 
 import pybamm
 
@@ -102,8 +102,8 @@ def __init__(
                     "len(x1) should equal y=shape[0], "
                     f"but x1.shape={x1.shape} and y.shape={y.shape}"
                 )
-        # children should be a list not a symbol
-        if isinstance(children, pybamm.Symbol):
+        # children should be a list not a symbol or a number
+        if isinstance(children, (pybamm.Symbol, numbers.Number)):
             children = [children]
         # Either a single x is provided and there is one child
         # or x is a 2-tuple and there are two children

tests/unit/test_expression_tree/test_interpolant.py

@@ -71,6 +71,11 @@ def test_interpolation(self):
                 interp.evaluate(y=np.array([2]))[:, 0], np.array([np.nan])
             )
 
+    def test_interpolation_float(self):
+        x = np.linspace(0, 1, 200)
+        interp = pybamm.Interpolant(x, 2 * x, 0.5)
+        assert interp.evaluate() == 1.0
+
     def test_interpolation_1_x_2d_y(self):
         x = np.linspace(0, 1, 200)
         y = np.tile(2 * x, (10, 1)).T