Fixed #80 - comma could start json list

lmformatenforcer/jsonschemaparser.py

@@ -615,15 +615,15 @@ def can_end(self) -> bool:
     def get_allowed_control_characters(self):
         num_items = self.num_items_seen
         is_on_top = self.root.context.active_parser.object_stack[-1] == self
-        if (not is_on_top) and self.root.last_non_whitespace_character != "[":
+        if (not is_on_top) and self.root.context.active_parser.last_non_whitespace_character != "[":
             # If there is an active parser above us, and the last character is not [, 
             # there is an active item parser on the stack that we did not count yet.
             num_items += 1
         control_characters = ""
         has_enough_items = self.min_items is None or num_items >= self.min_items
         can_add_another_item = self.max_items is None or num_items < self.max_items
 
-        if can_add_another_item:
+        if num_items > 0 and can_add_another_item:
             control_characters += ","
         if has_enough_items:
             control_characters += "]"

tests/test_jsonschemaparser.py

@@ -344,3 +344,13 @@ class EmptyListOKModel(BaseModel):
 
     no_strings = '{"num":1,"list_of_strings":[\n]}'
     _test_json_schema_parsing_with_string(no_strings, EmptyListOKModel.model_json_schema(), True)
+
+
+def test_comma_cannot_start_list():
+    class FlightRoute(BaseModel):
+        airports: List[str]
+    output_notok = """ { "airports": [,"name"] } """
+    output_ok = """ { "airports": ["name"] } """
+
+    _test_json_schema_parsing_with_string(output_ok, FlightRoute.model_json_schema(), True)
+    _test_json_schema_parsing_with_string(output_notok, FlightRoute.model_json_schema(), False)