您不能使用default参数执行此操作.***的选择是覆盖save方法:

You can't do this using the default argument. The best bet is to override the save method:

def save(self, *args, **kwargs):
    if not self.id and not self.printable:
        self.printable = self.link_fattura()
    return super(Fattura, self).save(*args, **kwargs)