code: def _ensure_array(self, key, value):
          
          if key not in self._json_dict:
              self._json_dict[key] = []
  
              self._size += 2             # brackets
              self._ensure_field(key)
  
          if len(self._json_dict[key]) > 0:
              # this array already has an entry, so add comma and space
              self._size += 2
  
          if isinstance(value, str):
              self._size += 2  # quotes
          self._size += len(str(value))
  
          self._json_dict[key].append(value)
docs: Ensure an array field