PayPal payment with OTP verification form

forms.py

from paypal.standard.forms import PayPalPaymentsForm as PPPF
from django import forms

class OTPForm(forms.Form):
  code = forms.CharField(label = "TOTP Code")

  def clean_code(self):
    code = self.cleaned_data['code']
    if not check_otp(code): # use libraries like onetimepass
      raise forms.ValidationError("Please enter a valid TOTP Code!")
    return code

  def get_payin_form(self, request):
    invoice = generate_some()
    form = PaypalPayInForm(invoice)
    pp_transfer = PayPalAllowedTransfer(
      user = request.user,
      invoice = form.invoice,
    )
    pp_transfer.save()
    return form

class PaypalPayInForm(PPPF):
  amount = forms.IntegerField()

  def __init__(self, invoice, *args, **kw):
    self.invoice = invoice
    pp_settings = self._default_initial()
    pp_settings.update(kwargs.get("initial", {}))
    kwargs["initial"] = pp_settings
  
  def _default_initial(self):
    return {
      "business": settings.PAYPAL_RECEIVER_EMAIL,
      "amount": "1",
      "currency_code": "EUR",
      "item_name": "your item name",
      "invoice": self.invoice,
      "notify_url": settings.BASE_URL + reverse("payin:paypal_notify"),
      "return_url": settings.BASE_URL + reverse("payin:paypal_return"),
      "cancel_return": settings.BASE_URL + reverse("payin:paypal_cancel"),  
    }

models.py

from django.contrib.auth.models import User
from django.db import models

class PayPalAllowedTransfer(models.Model):
  user = models.ForeignKey(User, editable = False)
  invoice = models.CharField("Invoice", max_length = 64, editable = False)
  created_at = models.DateTimeField("Creation Date", auto_now_add = True)
  used = models.BooleanField("Already used", default = False)

requirements.txt

django
django-paypal
onetimepass

signals.py

from paypal.standard.ipn.signals import payment_was_successful
from paypal.standard.models import ST_PP_COMPLETED

# this handles the PayPal IPNs, sender is an ipn object
def success(sender, **kwargs):
  invoice = sender.invoice
  if sender.payment_status != ST_PP_COMPLETED: return
  if PayPalAllowedTransfer.objects.filter(invoice = sender.invoice, used = False).exists():
    handle_payment_as_sucessfull(sender)
  else:
    flag_this_payment_as_not_allowed(sender)

payment_was_successful.connect(success)

views.py

from forms import OTPForm
from my_decorators import render_to # http://lincolnloop.com/blog/2008/may/10/getting-requestcontext-your-templates/
from django.views.decorators.http import require_POST

@render_to("paypal_template.html")
@require_POST
def paypal(request):
  form = PaypalPayInVerificationForm(request.POST)
  return {"form" : form.get_pain_form(request) if form.is_valid else form}