Django model field default from model method

I want to give a model field default value from the a model method.

How can i do that ?

when i try this code

Class Person(models.Model):
    def create_id(self):
        return os.urandom(12).encode('hex')

    name = models.CharField(max_length = 255)
    id = models.CharField(max_length = 255,default = self.create_id)

I get NameError: name 'self' is not defined.

If i remove the 'self' i get that 'create_id' needs 1 parameter.