old_contents: import random
  import string
  
  from django.db import models
  from django.utils import six
  from django.utils.encoding import python_2_unicode_compatible
  
  
  @python_2_unicode_compatible
  class MyWrapper(object):
      def __init__(self, value):
          self.value = value
  
      def __repr__(self):
          return "<%s: %s>" % (self.__class__.__name__, self.value)
  
      def __str__(self):
          return self.value
  
      def __eq__(self, other):
          if isinstance(other, self.__class__):
              return self.value == other.value
          return self.value == other
  
  class MyAutoField(six.with_metaclass(models.SubfieldBase, models.CharField)):
  
      def __init__(self, *args, **kwargs):
          kwargs['max_length'] = 10
          super(MyAutoField, self).__init__(*args, **kwargs)
  
      def pre_save(self, instance, add):
          value = getattr(instance, self.attname, None)
          if not value:
              value = MyWrapper(''.join(random.sample(string.lowercase, 10)))
              setattr(instance, self.attname, value)
          return value
  
      def to_python(self, value):
          if not value:
              return
          if not isinstance(value, MyWrapper):
              value = MyWrapper(value)
          return value
  
      def get_db_prep_save(self, value, connection):
          if not value:
              return
          if isinstance(value, MyWrapper):
              return six.text_type(value)
          return value
  
      def get_db_prep_value(self, value, connection, prepared=False):
          if not value:
              return
          if isinstance(value, MyWrapper):
              return six.text_type(value)
          return value
  
new_contents: import random
  import string
  
  from django.db import models
  from django.utils import six
  from django.utils.encoding import python_2_unicode_compatible
  
  
  @python_2_unicode_compatible
  class MyWrapper(object):
      def __init__(self, value):
          self.value = value
  
      def __repr__(self):
          return "<%s: %s>" % (self.__class__.__name__, self.value)
  
      def __str__(self):
          return self.value
  
      def __eq__(self, other):
          if isinstance(other, self.__class__):
              return self.value == other.value
          return self.value == other
  
  class MyAutoField(six.with_metaclass(models.SubfieldBase, models.CharField)):
  
      def __init__(self, *args, **kwargs):
          kwargs['max_length'] = 10
          super(MyAutoField, self).__init__(*args, **kwargs)
  
      def pre_save(self, instance, add):
          value = getattr(instance, self.attname, None)
          if not value:
              value = MyWrapper(''.join(random.sample(string.ascii_lowercase, 10)))
              setattr(instance, self.attname, value)
          return value
  
      def to_python(self, value):
          if not value:
              return
          if not isinstance(value, MyWrapper):
              value = MyWrapper(value)
          return value
  
      def get_db_prep_save(self, value, connection):
          if not value:
              return
          if isinstance(value, MyWrapper):
              return six.text_type(value)
          return value
  
      def get_db_prep_value(self, value, connection, prepared=False):
          if not value:
              return
          if isinstance(value, MyWrapper):
              return six.text_type(value)
          return value
  
subject: Use a name that exists in both python2 and 3,