Integrating a redirection-included method of payme

I am developing a shopping website using django-oscar framework, in fact I am using their sandbox site. I want to add payment to the checkout process, but the thing is, I am totally confused!

I've read this link : "Oscar's payment integration docs"

and I got the big picture stuff. I also read the file views.py in the checkout app, But I have a few questions which I couldn't find on the web.

My question is what are the methods/classes I should override or create to handle the following process :

after the user requests to make a payment, I should send a request to the bank and give them the required paramteres ( pay_request_parameters in code )

then, they will send an Id, confirming my access and then I should post that Id to an address and redirect the user to the bank's web page.

After the user has payed the bank successfully, they will inform me with a post to the call back url provided by me in the first step.

with that info, I should verify the payment's success and if it was successful, I should ask for a settlement from the bank, to send me the money.

Now the code I have does the first two steps, but I dont know how to handle the process after the redirection in sandbox. here's my code :

from oscar.apps.checkout import views
from oscar.apps.payment import models
from oscar.apps.payment.exceptions import *
import requests
import datetime

mellat_services_url = 'https://bpm.shaparak.ir/pgwchannel/services/pgw?wsdl'
start_pay_url = 'https://bpm.shaparak.ir/pgwchannel/startpay.mellat'
terminal_id = 'xxx'
username = 'xxx'
password = 'xxx'

# Subclass the core Oscar view so we can customise
class PaymentDetailsView(views.PaymentDetailsView):

    def handle_payment(self, order_number, total, **kwargs):
        # Talk to payment gateway.  If unsuccessful/error, raise a
        # PaymentError exception which we allow to percolate up to be caught
        # and handled by the core PaymentDetailsView.

        # mellat cycle start 
        local_date = str(datetime.date.today())[0:4] + str(datetime.date.today())[5:7] + str(datetime.date.today())[8:10]
        local_time = str(datetime.datetime.now().time())[0:2] + str(datetime.datetime.now().time())[3:5] + str(datetime.datetime.now().time())[6:8]
        # call bpPayRequest and get refId
        pay_request_parameters = {'terminalId': terminal_id, 'userName': username, 
                                'userPassword': password, 'orderId': order_number, 
                                'amount': total.incl_tax, 'localDate': local_date,
                                'localTime': local_time, 'additionalData': ""
                                'callBackUrl': 'mysite.com/checkout/preview/'} 

        pay_request_answer = requests.post(mellat_services_url, pay_request_parameters)

        if not pay_request_answer.split(",")[0] == 0:
            response_code = pay_request_answer.split(",")[0]
            if response_code[0] == '1':
                raise UnableToTakePayment()
            else: 
                raise PaymentError()

        requests.post(start_pay_url, pay_request_answer.split(",")[1])
        raise RedirectRequired(start_pay_url)

        # post the refId to bank and then redirect customer to the bank
        # apparently wait for the bank ( like for 10 mins ) to get the payment status
        # if the bank responded with success, the you verify the payment with a post to the bank
        # if everything was verified, tell the bank for a settlement

        # mellat cycle end

#The rest should be implemented but I dont know where I should put this
#All I know is that it should be done after the verification with the data
#sent from the bank. 

        reference = gateway.pre_auth(order_number, total.incl_tax, kwargs['bankcard'])

        # Payment successful! Record payment source
        source_type, __ = models.SourceType.objects.get_or_create(
            name="SomeGateway")
        source = models.Source(
            source_type=source_type,
            amount_allocated=total.incl_tax,
            reference=reference)
        self.add_payment_source(source)

        # Record payment event
        self.add_payment_event('pre-auth', total.incl_tax)

thanks in advance.

I had a similar problem, what I did was have the callBackUrl from the bank redirect to a view which is implemented something like this:

class CustomCheckoutDone(OrderPlacementMixin, RedirectView):
"""
here we verify payment was done and place the actual order
then redirect to thank you page
"""
permanent = False

def get_redirect_url(self, pk):
    basket = Basket.objects.get(pk=self.checkout_session.get_submitted_basket_id())
    basket.strategy = CustomStrategy()
    order_number = self.checkout_session.get_order_number()
    shipping_address = self.get_shipping_address(basket)
    shipping_method = self.get_shipping_method(basket, shipping_address)
    shipping_charge = shipping_method.calculate(basket)
    billing_address = self.get_billing_address(shipping_address)
    order_total = self.get_order_totals(basket, shipping_charge=shipping_charge)
    order_kwargs = {}
    # make sure payment was actually paid
    CustomPayment.objects.get(order_number=order_number, payed_sum=str(float(order_total.incl_tax)))
    user = self.request.user
    if not user.is_authenticated():
        order_kwargs['guest_email'] = self.checkout_session.get_guest_email()
    self.handle_order_placement(
        order_number, user, basket, shipping_address, shipping_method,
        shipping_charge, billing_address, order_total, **order_kwargs
    )
    return '/checkout/thank-you/'