Bug fix: Set default letter sender address on template creation

We spotted then when creating a new letter template it was not
having the default letter sender address added to it. This commit
fixes that bug by adding it if one exists.

Note, use of new walrus operator
https://docs.python.org/3/whatsnew/3.8.html#assignment-expressions

tests/app/template/test_rest.py

@@ -146,6 +146,70 @@ def test_create_a_new_template_for_a_service_adds_postage_for_letters_only(
     assert template.postage == expected_postage
 
 
+@pytest.mark.parametrize("has_a_non_default_letter_contact_only", [True, False])
+def test_create_letter_template_adds_no_sender_address_if_no_default_exists(
+    client, sample_service, has_a_non_default_letter_contact_only
+):
+    # If the service has no letter contacts, then the default sender address is 'not set' and it will be
+    # blank for the new template
+    # However they can have a single letter contact which is stored in the DB as not the default, which
+    # results in the default sender address being 'Blank' rather than 'not set'
+    # We test both of these situations where we expect the templates sender address to empty
+    if has_a_non_default_letter_contact_only:
+        create_letter_contact(sample_service, "Edinburgh, ED1 1AA", is_default=False)
+
+    data = {
+        'name': 'my template',
+        'template_type': 'letter',
+        'subject': "my subject",
+        'content': 'template <b>content</b>',
+        'service': str(sample_service.id),
+        'created_by': str(sample_service.users[0].id)
+    }
+
+    data = json.dumps(data)
+    auth_header = create_admin_authorization_header()
+
+    response = client.post(
+        '/service/{}/template'.format(sample_service.id),
+        headers=[('Content-Type', 'application/json'), auth_header],
+        data=data
+    )
+    assert response.status_code == 201
+    created_template = json.loads(response.get_data(as_text=True))['data']
+    assert created_template['service_letter_contact'] is None
+    assert created_template['reply_to'] is None
+    assert created_template['reply_to_text'] is None
+
+
+def test_create_letter_template_adds_default_sender_address_for_letters_when_exists(
+    client, sample_service
+):
+    letter_contact = create_letter_contact(sample_service, "Edinburgh, ED1 1AA", is_default=True)
+    data = {
+        'name': 'my template',
+        'template_type': 'letter',
+        'subject': "my subject",
+        'content': 'template <b>content</b>',
+        'service': str(sample_service.id),
+        'created_by': str(sample_service.users[0].id)
+    }
+
+    data = json.dumps(data)
+    auth_header = create_admin_authorization_header()
+
+    response = client.post(
+        '/service/{}/template'.format(sample_service.id),
+        headers=[('Content-Type', 'application/json'), auth_header],
+        data=data
+    )
+    assert response.status_code == 201
+    created_template = json.loads(response.get_data(as_text=True))['data']
+    assert created_template['service_letter_contact'] == str(letter_contact.id)
+    assert created_template['reply_to'] == str(letter_contact.id)
+    assert created_template['reply_to_text'] == "Edinburgh, ED1 1AA"
+
+
 def test_create_template_should_return_400_if_folder_is_for_a_different_service(
         client, sample_service
 ):