Postmark: Support both TemplateAlias and TemplateId as template_id

Accept either Postmark's template alias or numeric id for `template_id`.

CHANGELOG.rst

@@ -40,6 +40,12 @@ Breaking changes
   code depended on "temporary failure" showing up as "bounced" you will need to update it.
   (Thanks `@costela`_.)
 
+Features
+~~~~~~~~
+
+* **Postmark:** Allow either template alias (string) or numeric template id for
+  Anymail's `template_id` when sending with Postmark templates.
+
 Fixes
 ~~~~~
 

anymail/backends/postmark.py

@@ -139,7 +139,7 @@ class PostmarkPayload(RequestsPayload):
 
     def get_api_endpoint(self):
         batch_send = self.merge_data is not None and len(self.to_emails) > 1
-        if 'TemplateId' in self.data or 'TemplateModel' in self.data:
+        if 'TemplateAlias' in self.data or 'TemplateId' in self.data or 'TemplateModel' in self.data:
             if batch_send:
                 return "email/batchWithTemplates"
             else:
@@ -257,7 +257,11 @@ class PostmarkPayload(RequestsPayload):
         self.data["TrackOpens"] = track_opens
 
     def set_template_id(self, template_id):
-        self.data["TemplateId"] = template_id
+        try:
+            self.data["TemplateId"] = int(template_id)
+        except ValueError:
+            self.data["TemplateAlias"] = template_id
+
         # Subject, TextBody, and HtmlBody aren't allowed with TemplateId;
         # delete Django default subject and body empty strings:
         for field in ("Subject", "TextBody", "HtmlBody"):

docs/esps/postmark.rst

@@ -137,9 +137,14 @@ and :ref:`batch sending <batch-send>` with per-recipient merge data.
     :attr:`~anymail.message.AnymailMessage.merge_global_data` with Postmark.)
 
 To use a Postmark template, set the message's
-:attr:`~anymail.message.AnymailMessage.template_id` to the numeric Postmark
+:attr:`~anymail.message.AnymailMessage.template_id` to either the numeric Postmark
-"TemplateID" (*not* its name or "TemplateAlias"). You can find a template's
+"TemplateID" or its string "TemplateAlias" (which is *not* the template's name).
-numeric id near the top right in Postmark's template editor.
+You can find a template's numeric id near the top right in Postmark's template editor,
+and set the alias near the top right above the name.
+
+.. versionchanged:: 5.0
+
+    Earlier Anymail releases only allowed numeric template IDs.
 
 Supply the Postmark "TemplateModel" variables using Anymail's normalized
 :attr:`~anymail.message.AnymailMessage.merge_data` and
@@ -151,7 +156,7 @@ Supply the Postmark "TemplateModel" variables using Anymail's normalized
           # (subject and body come from the template, so don't include those)
           to=["alice@example.com", "Bob <bob@example.com>"]
       )
-      message.template_id = 80801  # Postmark template id
+      message.template_id = 80801  # Postmark template id or alias
       message.merge_data = {
           'alice@example.com': {'name': "Alice", 'order_no': "12345"},
           'bob@example.com': {'name': "Bob", 'order_no': "54321"},

tests/test_postmark_backend.py

@@ -385,6 +385,17 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         self.assertNotIn('HtmlBody', data)
         self.assertNotIn('TextBody', data)
 
+    def test_template_alias(self):
+        # Anymail template_id can be either Postmark TemplateId or TemplateAlias
+        message = AnymailMessage(
+            from_email='from@example.com', to=['to@example.com'],
+            template_id='welcome-message',
+        )
+        message.send()
+        self.assert_esp_called('/email/withTemplate/')
+        data = self.get_api_call_json()
+        self.assertEqual(data['TemplateAlias'], 'welcome-message')
+
     def test_merge_data(self):
         self.set_mock_response(raw=json.dumps([{
             "ErrorCode": 0,