allow downgradeable unicode characters in SMS templates

app/main/validators.py

@@ -1,7 +1,8 @@
 from wtforms import ValidationError
 from notifications_utils.template import Template
-from notifications_utils.gsm import get_non_gsm_characters
+from notifications_utils.gsm import get_non_gsm_compatible_characters
 
+from app import formatted_list
 from app.main._blacklisted_passwords import blacklisted_passwords
 from app.utils import (
     Spreadsheet,
@@ -54,8 +55,10 @@ class NoCommasInPlaceHolders:
 
 class OnlyGSMCharacters:
     def __call__(self, form, field):
-        non_gsm_characters = get_non_gsm_characters(field.data)
+        non_gsm_characters = sorted(list(get_non_gsm_compatible_characters(field.data)))
         if non_gsm_characters:
-            raise ValidationError('The following characters are not allowed in text messages: {}'.format(
+            raise ValidationError(
-                ', '.join(non_gsm_characters)
+                'You can’t use {} in text messages. They won’t show up properly on everyone’s phones.'.format(
-            ))
+                    formatted_list(non_gsm_characters, conjunction='or')
+                )
+            )

tests/app/main/test_validators.py

@@ -1,6 +1,6 @@
 import pytest
 from app.main.forms import RegisterUserForm, ServiceSmsSender
-from app.main.validators import ValidGovEmail, NoCommasInPlaceHolders
+from app.main.validators import ValidGovEmail, NoCommasInPlaceHolders, OnlyGSMCharacters
 from wtforms import ValidationError
 from unittest.mock import Mock
 
@@ -141,6 +141,21 @@ def test_for_commas_in_placeholders(
     NoCommasInPlaceHolders()(None, _gen_mock_field('Hello ((name))'))
 
 
+@pytest.mark.parametrize('msg', ['The quick brown fox', 'Thé “quick” bröwn fox\u200B'])
+def test_gsm_character_validation(client, msg):
+    OnlyGSMCharacters()(None, _gen_mock_field(msg))
+
+
+def test_non_gsm_character_validation(client):
+    with pytest.raises(ValidationError) as error:
+        OnlyGSMCharacters()(None, _gen_mock_field('∆ abc 📲 def 📵 ghi'))
+
+    assert str(error.value) == (
+        'You can’t use ‘∆’, ‘📲’ or ‘📵’ in text messages. '
+        'They won’t show up properly on everyone’s phones.'
+    )
+
+
 def test_sms_sender_form_validation(
     client,
     mock_get_user_by_email,