solution2.py

class Payment(models.Model):
    STATE_CHOICES = (
        ('started', 'Started'),
        ('captured', 'Captured'),
        ('completed', 'Completed'),
    )

    state = models.CharField(choices=STATE_CHOICES, default='started', max_length=16)

    def can_capture(self):
        return self.state == 'started'

    def can_complete(self):
        return self.state == 'captured'

    def capture(self):
        if self.can_capture():
            self.contact_payment_gateway()
            self.state = 'captured'
            self.save()
        else:
            raise ValueError()

    def complete(self):
        if self.can_complete():
            self.state = 'completed'
            self.save()
            self.notify_user()
        else:
            raise ValueError()