title: django set default value of a model field to a self attribute
body: <p>I have a model called Fattura, and I would like to set the default value of the field "printable" to a string that includes the value of the field "numero".</p>
  
  <p>But I have the error that link_fattura has less arguments, but if I add default=link_fattura(self) I have an error because self is not defined.</p>
  
  <p>How can I solve this issue?</p>
  
  <pre><code>class Fattura(models.Model):
          def link_fattura(self, *args, **kwargs):
                  return u"http://127.0.0.1:8000/fatture/%s/" % (self.numero)
          data = models.DateField()
          numero = models.CharField("Numero", max_length=3)
          fatturaProForma = models.ForeignKey(FatturaProForma)
          printable = models.CharField("Fattura stampabile", max_length=200, default=link_fattura)
          def __unicode__(self):
                  return u"%s %s" % (self.data, self.numero)
          class Meta:
                  verbose_name_plural = "Fatture"
                  ordering = ['data']
  </code></pre>