add a check for presence of size_dist

src/aero_mode.hpp

@@ -145,6 +145,10 @@ struct AeroMode {
             throw std::runtime_error("mass_frac value must be a list of single-element dicts");
         if (!InputJSONResource::unique_keys(mass_frac))
             throw std::runtime_error("mass_frac keys must be unique");
+        if (mode["mode_type"] == "sampled") {
+            if (mode.find("size_dist") == mode.end())
+                throw std::runtime_error("size_dist key must be set for mode_type=sampled");
+        }
     }
 
     static auto get_num_conc(const AeroMode &self){

tests/test_aero_mode.py

@@ -290,6 +290,20 @@ def test_segfault_case():  # TODO #319
         print(fishy_ctor_arg)
         ppmc.AeroMode(aero_data, fishy_ctor_arg)
 
+    @staticmethod
+    def test_sampled_without_size_dist():
+        # arrange
+        aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
+        fishy_ctor_arg = copy.deepcopy(AERO_MODE_CTOR_LOG_NORMAL)
+        fishy_ctor_arg["test_mode"]["mode_type"] = "sampled"
+
+        # act
+        with pytest.raises(Exception) as exc_info:
+            ppmc.AeroMode(aero_data, fishy_ctor_arg)
+
+        # assert
+        assert str(exc_info.value) == "size_dist key must be set for mode_type=sampled"
+
     @staticmethod
     def test_sampled():
         # arrange