PR for v.1.9.1

CHANGES.rst

@@ -1,4 +1,12 @@
 
+Release 1.9.1
+=========================================
+
+* **ENHANCEMENT:** Added stringified JSON serialization of callback functions when serializing via 
+  ``.to_json(for_export = True)``.
+
+----
+
 Release 1.9.0
 =========================================
 

highcharts_core/__version__.py

@@ -1 +1 @@
-__version__ = '1.9.0'
+__version__ = '1.9.1'

highcharts_core/metaclasses.py

@@ -317,7 +317,12 @@ def trim_dict(untrimmed: dict,
                 as_dict[key] = value
             # Callback Function
             elif checkers.is_type(value, 'CallbackFunction') and to_json:
-                continue
+                if not for_export:
+                    continue
+                elif value:
+                    trimmed_value = str(value)
+                    if trimmed_value and trimmed_value != 'None':
+                        as_dict[key] = trimmed_value
             # HighchartsMeta -> dict --> object
             elif value and hasattr(value, '_to_untrimmed_dict'):
                 untrimmed_value = value._to_untrimmed_dict()

highcharts_core/utility_classes/javascript_functions.py

@@ -138,6 +138,9 @@ def _to_untrimmed_dict(self, in_cls = None) -> dict:
         }
 
     def to_json(self, encoding = 'utf-8', for_export: bool = False):
+        if for_export:
+            return str(self)
+
         return None
 
     def to_js_literal(self,