Generate a unique code for an instance of a Django model on save

models.py

"""
This uses the DB to ensure uniqueness of the code. Better than checking separately 
with the DB and then saving, which introduces a race condition.
"""
class UserProfile(BaseModel):
    code = models.CharField(max_length=4,
                            db_index=True,
                            unique=True)

    def generate_code(self):
        """ Pick from 0-9, a-z EXCEPT O (capital o) because mistakes """
        self.code = ''.join(random.choice('0123456789ABCDEFGHIJKLMNPQRSTU'
                                          'VWXYZ') for i in range(4))
        return self

    def save(self, *args, **kwargs):
        """ Generate a unique code on first save """
        if not self.pk and not self.code:
            while True:
                try:
                    self.generate_code()
                    super(UserProfile, self).save(*args, **kwargs)
                except IntegrityError:
                    continue
                else:
                    break
        else:
            super(UserProfile, self).save(*args, **kwargs)