improve error diagnostics in python recipe (#1743)

python/recipe.cpp

@@ -14,6 +14,7 @@
 #include <arbor/morph/primitives.hpp>
 #include <arbor/recipe.hpp>
 
+#include "arbor/cable_cell_param.hpp"
 #include "conversion.hpp"
 #include "error.hpp"
 #include "event_generator.hpp"
@@ -70,6 +71,9 @@ static std::any convert_gprop(pybind11::object o) {
     if (o.is(pybind11::none())) {
         return {};
     }
+    if (!pybind11::isinstance<arb::cable_cell_global_properties>(o)) {
+        throw pyarb_error("recipe.global_properties must return a valid description of cable cell properties of type arbor.cable_global_properties");
+    }
     return pybind11::cast<arb::cable_cell_global_properties>(o);
 }
 

python/recipe.hpp

@@ -48,7 +48,6 @@ public:
     virtual pybind11::object global_properties(arb::cell_kind kind) const {
         return pybind11::none();
     };
-    //TODO: virtual pybind11::object global_properties(arb::cell_kind kind) const {return pybind11::none();};
 };
 
 class py_recipe_trampoline: public py_recipe {