SparkPost: work around json recipients problem

python-sparkpost generates a transmissions.send
payload which is now considered invalid by the API,
if you try to use both `cc` (or `bcc`) and the
`recipients` dict structure required for merge_data.

[Anymail had been generating that recipients dict
structure in all cases, for simplicity. Sometime
between 2016-06-07 and 2016-06-22, SparkPost
began rejecting that if it appeared in the `header_to`
constructed by python-sparkpost.]

anymail/backends/sparkpost.py

@@ -87,15 +87,20 @@ def init_payload(self):
     def get_api_params(self):
         # Compose recipients param from to_emails and merge_data (if any)
         recipients = []
-        for email in self.to_emails:
-            rcpt = {'address': {'email': email.email}}
-            if email.name:
-                rcpt['address']['name'] = email.name
-            try:
-                rcpt['substitution_data'] = self.merge_data[email.email]
-            except KeyError:
-                pass  # no merge_data or none for this recipient
-            recipients.append(rcpt)
+        if len(self.merge_data) > 0:
+            # Build JSON recipient structures
+            for email in self.to_emails:
+                rcpt = {'address': {'email': email.email}}
+                if email.name:
+                    rcpt['address']['name'] = email.name
+                try:
+                    rcpt['substitution_data'] = self.merge_data[email.email]
+                except KeyError:
+                    pass  # no merge_data or none for this recipient
+                recipients.append(rcpt)
+        else:
+            # Just use simple recipients list
+            recipients = [email.address for email in self.to_emails]
         if recipients:
             self.params['recipients'] = recipients
 

tests/test_sparkpost_backend.py

@@ -99,7 +99,7 @@ def test_send_mail(self):
         self.assertEqual(params['subject'], "Subject here")
         self.assertEqual(params['text'], "Here is the message.")
         self.assertEqual(params['from_email'], "from@example.com")
-        self.assertEqual(params['recipients'], [{'address': {'email': "to@example.com"}}])
+        self.assertEqual(params['recipients'], ["to@example.com"])
 
         self.assertEqual(self.get_send_api_key(), 'test_api_key')
 
@@ -118,10 +118,7 @@ def test_name_addr(self):
         params = self.get_send_params()
         self.assertEqual(params['from_email'], "From Name <from@example.com>")
         # We pre-parse the to-field emails (merge_data also gets attached there):
-        self.assertEqual(params['recipients'], [
-            {'address': {'email': 'to1@example.com', 'name': 'Recipient #1'}},
-            {'address': {'email': 'to2@example.com'}}
-        ])
+        self.assertEqual(params['recipients'], ['Recipient #1 <to1@example.com>', 'to2@example.com'])
         # We let python-sparkpost parse the other email fields:
         self.assertEqual(params['cc'], ['Carbon Copy <cc1@example.com>', 'cc2@example.com'])
         self.assertEqual(params['bcc'], ['Blind Copy <bcc1@example.com>', 'bcc2@example.com'])
@@ -141,10 +138,7 @@ def test_email_message(self):
         self.assertEqual(params['subject'], "Subject")
         self.assertEqual(params['text'], "Body goes here")
         self.assertEqual(params['from_email'], "from@example.com")
-        self.assertEqual(params['recipients'], [
-            {'address': {'email': 'to1@example.com'}},
-            {'address': {'email': 'to2@example.com', 'name': 'Also To'}}
-        ])
+        self.assertEqual(params['recipients'], ['to1@example.com', 'Also To <to2@example.com>'])
         self.assertEqual(params['bcc'], ['bcc1@example.com', 'Also BCC <bcc2@example.com>'])
         self.assertEqual(params['cc'], ['cc1@example.com', 'Also CC <cc2@example.com>'])
         self.assertEqual(params['custom_headers'], {