old_contents: from django.db import models
  from django import forms
  
  
  class ListTextField(models.TextField):
      description = "An field representing an array"
  
      __metaclass__ = models.SubfieldBase
  
      def __init__(self, separator=',', *args, **kwargs):
          self.separator = separator
          super(ListTextField, self).__init__(*args, **kwargs)
  
      def to_python(self, value):
          if isinstance(value, list):
              return value
  
          # We delete empty values
          return [x for x in value.split(self.separator) if x != '']
  
      def get_prep_value(self, value):
          return self.separator.join(value)
  
      def value_to_string(self, obj):
          value = self._get_val_from_obj(obj)
          return self.get_db_prep_value(value)
  
  
  from south.modelsinspector import add_introspection_rules
  add_introspection_rules([
      (
          [ListTextField],
          [],
          {
              "separator": ["separator", {"default": ","}],
          },
      ),
  ], ["^modular_blocks\.fields\.ListTextField"])
  
new_contents: from django.db import models
  from django import forms
  
  
  class ListTextField(models.TextField):
      description = "An field representing an array"
  
      __metaclass__ = models.SubfieldBase
  
      def __init__(self, separator=',', *args, **kwargs):
          self.separator = separator
          super(ListTextField, self).__init__(*args, **kwargs)
  
      def to_python(self, value):
          if isinstance(value, list):
              return value
  
          # We delete empty values
          return [x for x in value.split(self.separator) if x != '']
  
      def get_prep_value(self, value):
          return self.separator.join(value)
  
      def value_to_string(self, obj):
          value = self._get_val_from_obj(obj)
          return self.get_prep_value(value)
  
  
  from south.modelsinspector import add_introspection_rules
  add_introspection_rules([
      (
          [ListTextField],
          [],
          {
              "separator": ["separator", {"default": ","}],
          },
      ),
  ], ["^modular_blocks\.fields\.ListTextField"])
  
subject: use get_prep_value instead of get_db_prep_value