Hexadecimal Primary Key in Django Model

The following snippet of code will make the primary key in a Django model to be a hexadecimal string of 8 characters instead of an integer.

I wrote this earlier today, and it works fine, but I have a bad feeling about it. I don't know why yet, but something doesn't feel right. Nevertheless, I am putting it here as a note to myself. You should probably not use it. If you do, note that if you exhaust the possible IDs, generate_id() will recurse forever.

import os
from binascii import hexlify
from django.db import models

# Create your models here.
class Person(models.Model):
    '''Hold a Person object'''

    def generate_id():
        '''Generate an 8-character long hexadecimal ID'''
        possible = hexlify(os.urandom(4))
            # if this possible ID exists, run again:
            return self.generate_id()
            return possible

    ID = models.CharField(
        max_length = 8,
        primary_key = True,
        editable = False,
        default = generate_id
    first_name = models.CharField(max_length=240)