Resend: support batch send

Add support for `merge_metadata`
and new Resend email/batch API.

CHANGELOG.rst

@@ -35,6 +35,8 @@ Features
 
 * **Brevo:** Add support for batch sending
   (`docs <https://anymail.dev/en/latest/esps/brevo/#batch-sending-merge-and-esp-templates>`__).
+* **Resend:** Add support for batch sending
+  (`docs <https://anymail.dev/en/latest/esps/resend/#batch-sending-merge-and-esp-templates>`__).
 
 
 v10.2

anymail/backends/resend.py

@@ -3,6 +3,7 @@ from email.charset import QP, Charset
 from email.header import decode_header, make_header
 from email.headerregistry import Address
 
+from ..exceptions import AnymailRequestsAPIError
 from ..message import AnymailRecipientStatus
 from ..utils import (
     BASIC_NUMERIC_TYPES,
@@ -56,10 +57,24 @@ class EmailBackend(AnymailRequestsBackend):
         return ResendPayload(message, defaults, self)
 
     def parse_recipient_status(self, response, payload, message):
-        # Resend provides single message id, no other information.
-        # Assume "queued".
         parsed_response = self.deserialize_json_response(response, payload, message)
+        try:
             message_id = parsed_response["id"]
+            message_ids = None
+        except (KeyError, TypeError):
+            # Batch send?
+            try:
+                message_id = None
+                message_ids = [item["id"] for item in parsed_response["data"]]
+            except (KeyError, TypeError) as err:
+                raise AnymailRequestsAPIError(
+                    "Invalid Resend API response format",
+                    email_message=message,
+                    payload=payload,
+                    response=response,
+                    backend=self,
+                ) from err
+
         recipient_status = CaseInsensitiveCasePreservingDict(
             {
                 recip.addr_spec: AnymailRecipientStatus(
@@ -68,12 +83,21 @@ class EmailBackend(AnymailRequestsBackend):
                 for recip in payload.recipients
             }
         )
+        if message_ids:
+            # batch send: ids are in same order as to_recipients
+            for recip, message_id in zip(payload.to_recipients, message_ids):
+                recipient_status[recip.addr_spec] = AnymailRecipientStatus(
+                    message_id=message_id, status="queued"
+                )
         return dict(recipient_status)
 
 
 class ResendPayload(RequestsPayload):
     def __init__(self, message, defaults, backend, *args, **kwargs):
         self.recipients = []  # for parse_recipient_status
+        self.to_recipients = []  # for parse_recipient_status
+        self.metadata = {}
+        self.merge_metadata = {}
         headers = kwargs.pop("headers", {})
         headers["Authorization"] = "Bearer %s" % backend.api_key
         headers["Content-Type"] = "application/json"
@@ -81,10 +105,33 @@ class ResendPayload(RequestsPayload):
         super().__init__(message, defaults, backend, headers=headers, *args, **kwargs)
 
     def get_api_endpoint(self):
+        if self.is_batch():
+            return "emails/batch"
         return "emails"
 
     def serialize_data(self):
-        return self.serialize_json(self.data)
+        payload = self.data
+        if self.is_batch():
+            # Burst payload across to addresses
+            to_emails = self.data.pop("to", [])
+            payload = []
+            for to_email, to in zip(to_emails, self.to_recipients):
+                data = self.data.copy()
+                data["to"] = [to_email]  # formatted for Resend (w/ workarounds)
+                if to.addr_spec in self.merge_metadata:
+                    # Merge global metadata with any per-recipient metadata.
+                    recipient_metadata = self.metadata.copy()
+                    recipient_metadata.update(self.merge_metadata[to.addr_spec])
+                    if "headers" in data:
+                        data["headers"] = data["headers"].copy()
+                    else:
+                        data["headers"] = {}
+                    data["headers"]["X-Metadata"] = self.serialize_json(
+                        recipient_metadata
+                    )
+                payload.append(data)
+
+        return self.serialize_json(payload)
 
     #
     # Payload construction
@@ -147,6 +194,8 @@ class ResendPayload(RequestsPayload):
             field = recipient_type
             self.data[field] = [self._resend_email_address(email) for email in emails]
             self.recipients += emails
+            if recipient_type == "to":
+                self.to_recipients = emails
 
     def set_subject(self, subject):
         self.data["subject"] = subject
@@ -206,6 +255,7 @@ class ResendPayload(RequestsPayload):
         self.data.setdefault("headers", {})["X-Metadata"] = self.serialize_json(
             metadata
         )
+        self.metadata = metadata  # may be needed for batch send in serialize_data
 
     # Resend doesn't support delayed sending
     # def set_send_at(self, send_at):
@@ -223,9 +273,16 @@ class ResendPayload(RequestsPayload):
     # (Their template feature is rendered client-side,
     # using React in node.js.)
     # def set_template_id(self, template_id):
-    # def set_merge_data(self, merge_data):
     # def set_merge_global_data(self, merge_global_data):
-    # def set_merge_metadata(self, merge_metadata):
+
+    def set_merge_data(self, merge_data):
+        # Empty merge_data is a request to use batch send;
+        # any other merge_data is unsupported.
+        if any(recipient_data for recipient_data in merge_data.values()):
+            self.unsupported_feature("merge_data")
+
+    def set_merge_metadata(self, merge_metadata):
+        self.merge_metadata = merge_metadata  # late bound in serialize_data
 
     def set_esp_extra(self, extra):
         self.data.update(extra)

docs/esps/esp-feature-matrix.csv

@@ -2,7 +2,7 @@ Email Service Provider,:ref:`amazon-ses-backend`,:ref:`brevo-backend`,:ref:`mail
 .. rubric:: :ref:`Anymail send options <anymail-send-options>`,,,,,,,,,,,
 :attr:`~AnymailMessage.envelope_sender`,Yes,No,No,Domain only,Yes,Domain only,Yes,No,No,No,Yes
 :attr:`~AnymailMessage.metadata`,Yes,Yes,No,Yes,Yes,Yes,No,Yes,Yes,Yes,Yes
-:attr:`~AnymailMessage.merge_metadata`,No,Yes,No,Yes,Yes,Yes,No,Yes,No,Yes,Yes
+:attr:`~AnymailMessage.merge_metadata`,No,Yes,No,Yes,Yes,Yes,No,Yes,Yes,Yes,Yes
 :attr:`~AnymailMessage.send_at`,No,Yes,Yes,Yes,No,Yes,No,No,No,Yes,Yes
 :attr:`~AnymailMessage.tags`,Yes,Yes,Yes,Yes,Max 1 tag,Yes,Max 1 tag,Max 1 tag,Yes,Yes,Max 1 tag
 :attr:`~AnymailMessage.track_clicks`,No,No,Yes,Yes,Yes,Yes,No,Yes,No,Yes,Yes

docs/esps/resend.rst

@@ -176,16 +176,6 @@ anyway---see :ref:`unsupported-features`.
   tracking webhook using :ref:`esp_event <resend-esp-event>`. (The linked
   sections below include examples.)
 
-**No stored templates or batch sending**
-  Resend does not currently offer ESP stored templates or merge capabilities,
-  including Anymail's
-  :attr:`~anymail.message.AnymailMessage.merge_data`,
-  :attr:`~anymail.message.AnymailMessage.merge_global_data`,
-  :attr:`~anymail.message.AnymailMessage.merge_metadata`, and
-  :attr:`~anymail.message.AnymailMessage.template_id` features.
-  (Resend's current template feature is only supported in node.js,
-  using templates that are rendered in their API client.)
-
 **No click/open tracking overrides**
   Resend does not support :attr:`~anymail.message.AnymailMessage.track_clicks`
   or :attr:`~anymail.message.AnymailMessage.track_opens`. Its
@@ -242,6 +232,47 @@ values directly to Resend's `send-email API`_. Example:
       }
 
 
+.. _resend-templates:
+
+Batch sending/merge and ESP templates
+-------------------------------------
+
+.. versionadded:: 10.3
+
+    Support for batch sending with
+    :attr:`~anymail.message.AnymailMessage.merge_metadata`.
+
+Resend supports :ref:`batch sending <batch-send>` (where each *To*
+recipient sees only their own email address). It also supports
+per-recipient metadata with batch sending.
+
+Set Anymail's normalized :attr:`~anymail.message.AnymailMessage.merge_metadata`
+attribute to use Resend's batch-send API:
+
+  .. code-block:: python
+
+      message = EmailMessage(
+          to=["alice@example.com", "Bob <bob@example.com>"],
+          from_email="...", subject="...", body="..."
+      )
+      message.merge_metadata = {
+          'alice@example.com': {'user_id': "12345"},
+          'bob@example.com': {'user_id': "54321"},
+      }
+
+Resend does not currently offer :ref:`ESP stored templates <esp-stored-templates>`
+or merge capabilities, so does not support Anymail's
+:attr:`~anymail.message.AnymailMessage.merge_data`,
+:attr:`~anymail.message.AnymailMessage.merge_global_data`, or
+:attr:`~anymail.message.AnymailMessage.template_id` message attributes.
+(Resend's current template feature is only supported in node.js,
+using templates that are rendered in their API client.)
+
+(Setting :attr:`~anymail.message.AnymailMessage.merge_data` to an empty
+dict will also invoke batch send, but trying to supply merge data for
+any recipient will raise an :exc:`~anymail.exceptions.AnymailUnsupportedFeature` error.)
+
+
 .. _resend-webhooks:
 
 Status tracking webhooks

tests/test_resend_backend.py

@@ -14,7 +14,7 @@ from anymail.exceptions import (
     AnymailSerializationError,
     AnymailUnsupportedFeature,
 )
-from anymail.message import attach_inline_image_file
+from anymail.message import AnymailMessage, attach_inline_image_file
 
 from .mock_requests_backend import (
     RequestsBackendMockAPITestCase,
@@ -445,6 +445,94 @@ class ResendBackendAnymailFeatureTests(ResendBackendMockAPITestCase):
             },
         )
 
+    _mock_batch_response = {
+        "data": [
+            {"id": "ae2014de-c168-4c61-8267-70d2662a1ce1"},
+            {"id": "faccb7a5-8a28-4e9a-ac64-8da1cc3bc1cb"},
+        ]
+    }
+
+    def test_merge_data(self):
+        self.message.merge_data = {"to@example.com": {"customer_id": 3}}
+        with self.assertRaisesMessage(AnymailUnsupportedFeature, "merge_data"):
+            self.message.send()
+
+    def test_empty_merge_data(self):
+        # `merge_data = {}` triggers batch send
+        self.set_mock_response(json_data=self._mock_batch_response)
+        message = AnymailMessage(
+            from_email="from@example.com",
+            to=["alice@example.com", "Bob <bob@example.com>"],
+            cc=["cc@example.com"],
+            merge_data={
+                "alice@example.com": {},
+                "bob@example.com": {},
+            },
+        )
+        message.send()
+        self.assert_esp_called("/emails/batch")
+        data = self.get_api_call_json()
+        self.assertEqual(len(data), 2)
+        self.assertEqual(data[0]["to"], ["alice@example.com"])
+        self.assertEqual(data[1]["to"], ["Bob <bob@example.com>"])
+
+        recipients = message.anymail_status.recipients
+        self.assertEqual(recipients["alice@example.com"].status, "queued")
+        self.assertEqual(
+            recipients["alice@example.com"].message_id,
+            "ae2014de-c168-4c61-8267-70d2662a1ce1",
+        )
+        self.assertEqual(recipients["bob@example.com"].status, "queued")
+        self.assertEqual(
+            recipients["bob@example.com"].message_id,
+            "faccb7a5-8a28-4e9a-ac64-8da1cc3bc1cb",
+        )
+        # No message_id for cc/bcc recipients in a batch send
+        self.assertEqual(recipients["cc@example.com"].status, "queued")
+        self.assertIsNone(recipients["cc@example.com"].message_id)
+
+    def test_merge_metadata(self):
+        self.set_mock_response(json_data=self._mock_batch_response)
+        message = AnymailMessage(
+            from_email="from@example.com",
+            to=["alice@example.com", "Bob <bob@example.com>"],
+            merge_metadata={
+                "alice@example.com": {"order_id": 123, "tier": "premium"},
+                "bob@example.com": {"order_id": 678},
+            },
+            metadata={"notification_batch": "zx912"},
+        )
+        message.send()
+
+        # merge_metadata forces batch send API:
+        self.assert_esp_called("/emails/batch")
+
+        data = self.get_api_call_json()
+        self.assertEqual(len(data), 2)
+        self.assertEqual(data[0]["to"], ["alice@example.com"])
+        # metadata and merge_metadata[recipient] are combined:
+        self.assertEqual(
+            json.loads(data[0]["headers"]["X-Metadata"]),
+            {"order_id": 123, "tier": "premium", "notification_batch": "zx912"},
+        )
+        self.assertEqual(data[1]["to"], ["Bob <bob@example.com>"])
+        self.assertEqual(
+            json.loads(data[1]["headers"]["X-Metadata"]),
+            {"order_id": 678, "notification_batch": "zx912"},
+        )
+
+        recipients = message.anymail_status.recipients
+        self.assertEqual(recipients["alice@example.com"].status, "queued")
+        self.assertEqual(
+            recipients["alice@example.com"].message_id,
+            "ae2014de-c168-4c61-8267-70d2662a1ce1",
+        )
+        self.assertEqual(recipients["bob@example.com"].status, "queued")
+        self.assertEqual(
+            recipients["bob@example.com"].message_id,
+            "faccb7a5-8a28-4e9a-ac64-8da1cc3bc1cb",
+        )
+
     def test_track_opens(self):
         self.message.track_opens = True
         with self.assertRaisesMessage(AnymailUnsupportedFeature, "track_opens"):

tests/test_resend_integration.py

@@ -86,6 +86,37 @@ class ResendBackendIntegrationTests(AnymailTestMixin, SimpleTestCase):
             len(message.anymail_status.message_id), 0
         )  # non-empty string
 
+    def test_batch_send(self):
+        # merge_metadata or merge_data will use batch send API
+        message = AnymailMessage(
+            subject="Anymail Resend batch sendintegration test",
+            body="This is the text body",
+            from_email=self.from_email,
+            to=["test+to1@anymail.dev", '"Recipient 2" <test+to2@anymail.dev>'],
+            metadata={"meta1": "simple string", "meta2": 2},
+            merge_metadata={
+                "test+to1@anymail.dev": {"meta3": "recipient 1"},
+                "test+to2@anymail.dev": {"meta3": "recipient 2"},
+            },
+            tags=["tag 1", "tag 2"],
+        )
+        message.attach_alternative("<p>HTML content</p>", "text/html")
+        message.attach("attachment1.txt", "Here is some\ntext for you", "text/plain")
+
+        message.send()
+        # Resend always queues:
+        self.assertEqual(message.anymail_status.status, {"queued"})
+        recipient_status = message.anymail_status.recipients
+        self.assertEqual(recipient_status["test+to1@anymail.dev"].status, "queued")
+        self.assertEqual(recipient_status["test+to2@anymail.dev"].status, "queued")
+        self.assertRegex(recipient_status["test+to1@anymail.dev"].message_id, r".+")
+        self.assertRegex(recipient_status["test+to2@anymail.dev"].message_id, r".+")
+        # Each recipient gets their own message_id:
+        self.assertNotEqual(
+            recipient_status["test+to1@anymail.dev"].message_id,
+            recipient_status["test+to2@anymail.dev"].message_id,
+        )
+
     @unittest.skip("Resend has stopped responding to bad/missing API keys (12/2023)")
     @override_settings(ANYMAIL_RESEND_API_KEY="Hey, that's not an API key!")
     def test_invalid_api_key(self):