solution2_retry.py

class Payment(models.Model):
    STATE_CHOICES = (
        ('started', 'Started'),
        ('captured', 'Captured'),
        ('completed', 'Completed'),
        # we introduce a new "incomplete" state
        ('incomplete', 'Incomplete'),
    )

    def can_complete(self):
        # We can complete both captured and incomplete payments
        return self.state in ('captured', 'incomplete')

    def can_retry(self):
        # We can only retry incomplete payments
        return self.state == 'incomplete'

    def capture(self):
        if self.can_capture():
            try:
                self.contact_payment_gateway()
            except PaymentGatewayException:
                # We can mark a payment as incomplete if we couldn't 
                # capture it from the payment gateway
                self.state = 'incomplete'
                self.save()
                return

            self.state = 'captured'
            self.save()
        else:
            raise ValueError()

    def retry(self):
        # We can call retry() to complete the payment if we it is incomplete
        if self.can_retry():
            self.complete()