#917 fix function that returns float

CHANGELOG.md

@@ -18,6 +18,7 @@
 
 ## Bug fixes
 
+-   Fixed bug raised if function returns a scalar ([#919](https://github.com/pybamm-team/PyBaMM/pull/919))
 -   Updated Getting started notebook 2 ([#903](https://github.com/pybamm-team/PyBaMM/pull/903))
 -   Reformatted external circuit submodels ([#879](https://github.com/pybamm-team/PyBaMM/pull/879))
 -   Some bug fixes to generalize specifying models that aren't battery models, see [#846](https://github.com/pybamm-team/PyBaMM/issues/846)

pybamm/parameters/parameter_values.py

@@ -524,6 +524,9 @@ def _process_symbol(self, symbol):
                 # return differentiated function
                 new_diff_variable = self.process_symbol(symbol.diff_variable)
                 function_out = function.diff(new_diff_variable)
+            # Convert possible float output to a pybamm scalar
+            if isinstance(function_out, numbers.Number):
+                return pybamm.Scalar(function_out)
             # Process again just to be sure
             return self.process_symbol(function_out)
 

tests/unit/test_parameters/test_parameter_values.py

@@ -300,6 +300,7 @@ def test_process_function_parameter(self):
                 "a": 3,
                 "func": pybamm.load_function("process_symbol_test_function.py"),
                 "const": 254,
+                "float_func": lambda x: 42,
             }
         )
         a = pybamm.InputParameter("a")
@@ -320,6 +321,11 @@ def test_process_function_parameter(self):
         processed_diff_func = parameter_values.process_symbol(diff_func)
         self.assertEqual(processed_diff_func.evaluate(u={"a": 3}), 123)
 
+        # function parameter that returns a python float
+        func = pybamm.FunctionParameter("float_func", a)
+        processed_func = parameter_values.process_symbol(func)
+        self.assertEqual(processed_func.evaluate(), 42)
+
         # function itself as input (different to the variable being an input)
         parameter_values = pybamm.ParameterValues({"func": "[input]"})
         a = pybamm.Scalar(3)