fix paypal api url joining

payments/backends/paypal.py

@@ -9,6 +9,14 @@ import requests
 from .base import BackendBase
 
 
+def urljoin(a, b):                                                                                                                                                                                                                                                              
+    if b.startswith('/') and a.endswith('/'):                                                                                                                                                                                                                                   
+        return a + b[1:]                                                                                                                                                                                                                                                        
+    if b.startswith('/') or a.endswith('/'):                                                                                                                                                                                                                                    
+        return a + b                                                                                                                                                                                                                                                            
+    return a + "/" + b
+
+
 class PaypalBackend(BackendBase):
     backend_id = 'paypal'
     backend_verbose_name = _("PayPal")
@@ -60,7 +68,7 @@ class PaypalBackend(BackendBase):
                                    "It can take up to a few minutes...")
         payment.save()
 
-        return redirect(self.api_base + '/cgi-bin/webscr?' + urlencode(params))
+        return redirect(urljoin(self.api_base, '/cgi-bin/webscr?' + urlencode(params)))
 
     def new_subscription(self, rps):
         months = {
@@ -91,7 +99,7 @@ class PaypalBackend(BackendBase):
 
         rps.save()
 
-        return redirect(self.api_base + '/cgi-bin/webscr?' + urlencode(params))
+        return redirect(urljoin(self.api_base, '/cgi-bin/webscr?' + urlencode(params)))
 
     def handle_verified_callback(self, payment, params):
         if self.test and params['test_ipn'] != '1':
@@ -165,7 +173,7 @@ class PaypalBackend(BackendBase):
         return True
 
     def verify_ipn(self, request):
-        v_url = self.api_base + '/cgi-bin/webscr?cmd=_notify-validate'
+        v_url = urljoin(self.api_base, '/cgi-bin/webscr?cmd=_notify-validate')
         v_req = urlopen(v_url, data=request.body, timeout=5)
         v_res = v_req.read()
         return v_res == b'VERIFIED'