code: def _get_default_letters(model_admin=None):
      
      from django.conf import settings
      import string
      default_ltrs = string.digits + string.ascii_uppercase
      default_letters = getattr(settings, 'DEFAULT_ALPHABET', default_ltrs)
      if model_admin and hasattr(model_admin, 'DEFAULT_ALPHABET'):
          default_letters = model_admin.DEFAULT_ALPHABET
      if callable(default_letters):
          return set(default_letters())
      elif isinstance(default_letters, str):
          return set([x for x in default_letters])
      elif isinstance(default_letters, str):
          return set([x for x in default_letters.decode('utf8')])
      elif isinstance(default_letters, (tuple, list)):
          return set(default_letters)
docs: Returns the set of letters defined in the configuration variable
      DEFAULT_ALPHABET. DEFAULT_ALPHABET can be a callable, string, tuple, or
      list and returns a set.
  
      If a ModelAdmin class is passed, it will look for a DEFAULT_ALPHABET
      attribute and use it instead.