SparkPost: support merge_headers

anymail/backends/sparkpost.py

@@ -242,6 +242,36 @@ def set_merge_metadata(self, merge_metadata):
             if to_email in merge_metadata:
                 recipient["metadata"] = merge_metadata[to_email]
 
+    def set_merge_headers(self, merge_headers):
+        def header_var(field):
+            return "Header__" + field.title().replace("-", "_")
+
+        merge_header_fields = set()
+
+        for recipient in self.data["recipients"]:
+            to_email = recipient["address"]["email"]
+            if to_email in merge_headers:
+                recipient_headers = merge_headers[to_email]
+                recipient.setdefault("substitution_data", {}).update(
+                    {header_var(key): value for key, value in recipient_headers.items()}
+                )
+                merge_header_fields.update(recipient_headers.keys())
+
+        if merge_header_fields:
+            headers = self.data.setdefault("content", {}).setdefault("headers", {})
+            # Global substitution_data supplies defaults for defined headers:
+            self.data.setdefault("substitution_data", {}).update(
+                {
+                    header_var(field): headers[field]
+                    for field in merge_header_fields
+                    if field in headers
+                }
+            )
+            # Indirect merge_headers through substitution_data:
+            headers.update(
+                {field: "{{%s}}" % header_var(field) for field in merge_header_fields}
+            )
+
     def set_send_at(self, send_at):
         try:
             start_time = send_at.replace(microsecond=0).isoformat()

tests/test_sparkpost_backend.py

@@ -605,6 +605,58 @@ def test_merge_metadata(self):
         )
         self.assertEqual(data["metadata"], {"notification_batch": "zx912"})
 
+    def test_merge_headers(self):
+        self.set_mock_result(accepted=2)
+        self.message.to = ["[email protected]", "Bob <[email protected]>"]
+        self.message.extra_headers = {
+            "X-Custom-1": "custom 1",
+            "X-Custom-2": "custom 2 (default)",
+        }
+        self.message.merge_headers = {
+            "[email protected]": {
+                "X-Custom-2": "custom 2 alice",
+                "X-Custom-3": "custom 3 alice",
+            },
+            "[email protected]": {"X-Custom-2": "custom 2 bob"},
+        }
+
+        self.message.send()
+        data = self.get_api_call_json()
+        recipients = data["recipients"]
+        self.assertEqual(len(recipients), 2)
+        self.assertEqual(recipients[0]["address"]["email"], "[email protected]")
+        self.assertEqual(
+            recipients[0]["substitution_data"],
+            {
+                "Header__X_Custom_2": "custom 2 alice",
+                "Header__X_Custom_3": "custom 3 alice",
+            },
+        )
+        self.assertEqual(recipients[1]["address"]["email"], "[email protected]")
+        self.assertEqual(
+            recipients[1]["substitution_data"],
+            {
+                "Header__X_Custom_2": "custom 2 bob",
+            },
+        )
+        # Indirect merge_headers through template substitutions:
+        self.assertEqual(
+            data["content"]["headers"],
+            {
+                "X-Custom-1": "custom 1",  # (not a merge_header, value unchanged)
+                "X-Custom-2": "{{Header__X_Custom_2}}",
+                "X-Custom-3": "{{Header__X_Custom_3}}",
+            },
+        )
+        # Defaults for merge_headers in global substitution_data:
+        self.assertEqual(
+            data["substitution_data"],
+            {
+                "Header__X_Custom_2": "custom 2 (default)",
+                # No default specified for X-Custom-3; SparkPost will use empty string
+            },
+        )
+
     def test_default_omits_options(self):
         """Make sure by default we don't send any ESP-specific options.
 

tests/test_sparkpost_integration.py

@@ -116,6 +116,19 @@ def test_merge_data(self):
                 "[email protected]": {"value": "two"},
             },
             merge_global_data={"global": "global_value"},
+            merge_metadata={
+                "[email protected]": {"meta1": "one"},
+                "[email protected]": {"meta1": "two"},
+            },
+            headers={
+                "X-Custom": "custom header default",
+            },
+            merge_headers={
+                # (Note that SparkPost doesn't support custom List-Unsubscribe headers)
+                "[email protected]": {
+                    "X-Custom": "custom header one",
+                },
+            },
         )
         message.send()
         recipient_status = message.anymail_status.recipients