Add expected_sopn_date for ballots

requirements/base.txt

@@ -31,6 +31,7 @@ djangorestframework-jsonp==1.0.2
 djangorestframework-csv==2.1.1
 djangorestframework==3.14
 drf-yasg==1.21.5
+uk-election-timetables==2.4.1
 factory-boy==3.2.1
 Faker==19.2.0
 freezegun==1.2.2

ynr/apps/candidates/helpers/helpers.py

@@ -0,0 +1,20 @@
+from uk_election_timetables.calendars import Country
+from uk_election_timetables.election_ids import from_election_id
+
+
+def get_election_timetable(slug, territory):
+    country = {
+        "ENG": Country.ENGLAND,
+        "WLS": Country.WALES,
+        "SCT": Country.SCOTLAND,
+        "NIR": Country.NORTHERN_IRELAND,
+    }
+
+    if slug.startswith("local") and territory not in country:
+        return None
+
+    try:
+        return from_election_id(slug, country[territory])
+
+    except BaseException:
+        return None

ynr/apps/candidates/models/popolo_extra.py

@@ -1,6 +1,7 @@
 import datetime
 import hashlib
 
+from candidates.helpers.helpers import get_election_timetable
 from candidates.models import LoggedAction
 from candidates.models.auth import TRUSTED_TO_LOCK_GROUP_NAME
 from candidates.models.db import ActionType
@@ -558,6 +559,16 @@ def is_welsh_run(self):
 
         return self.tags.get("NUTS1", {}).get("key") == "UKL"
 
+    @property
+    def expected_sopn_date(self):
+        try:
+            return get_election_timetable(
+                self.ballot_paper_id, self.post.territory_code
+            ).sopn_publish_date
+
+        except AttributeError:
+            return None
+
 
 class PartySet(models.Model):
     slug = models.CharField(max_length=256, unique=True)