Give a more informative error for JSON not serializable.

Author: rmarren1

dash/dash.py

@@ -521,11 +521,22 @@ def add_context(*args, **kwargs):
                     }
                 }
 
-                return flask.Response(
-                    json.dumps(response,
-                               cls=plotly.utils.PlotlyJSONEncoder),
-                    mimetype='application/json'
-                )
+                try:
+                    return flask.Response(
+                        json.dumps(response,
+                                   cls=plotly.utils.PlotlyJSONEncoder),
+                        mimetype='application/json'
+                    )
+                except TypeError:
+                    raise exceptions.ReturnValueNotJSONSerializable('''
+                        Callback for property `{:s}` from component id `{:s}`
+                        returned a value `{}` of type `{:s}`,
+                        which is not JSON serializable.
+                    '''.format(
+                        output.component_property,
+                        output.component_id,
+                        output_value,
+                        type(output_value).__name__).replace('    ', ''))
 
             self.callback_map[callback_id]['callback'] = add_context
 

dash/exceptions.py

@@ -44,3 +44,7 @@ class CantHaveMultipleOutputs(CallbackException):
 
 class PreventUpdate(CallbackException):
     pass
+
+
+class ReturnValueNotJSONSerializable(CallbackException):
+    pass