sys.exit rather than throwing an assertion error

Assertions should only be used in tests - they can be disabled at
runtime by setting the python flag -O (though I don't believe we use
that flag under normal circumstances).

also clean up test asserts - mock_redis is the redis object, so its
`called` property will always be false, because we never say
`redis_store()`. Rather, we should use the `mock_calls` property to
see all calls to all of its children

app/commands.py

@@ -1,3 +1,4 @@
+import sys
 import functools
 import uuid
 from datetime import datetime, timedelta
@@ -504,9 +505,11 @@ def populate_redis_template_usage(service_id, day):
     Recalculate and replace the stats in redis for a day.
     To be used if redis data is lost for some reason.
     """
-    # the day variable is set to midnight of that day
+    if not current_app.config['REDIS_ENABLED']:
-    assert current_app.config['REDIS_ENABLED']
+        current_app.logger.error('Cannot populate redis template usage - redis not enabled')
+        sys.exit(1)
 
+    # the day variable is set by click to be midnight of that day
     start_time = get_london_midnight_in_utc(day)
     end_time = get_london_midnight_in_utc(day + timedelta(days=1))
 
@@ -533,5 +536,6 @@ def populate_redis_template_usage(service_id, day):
         redis_store.set_hash_and_expire(
             key,
             usage,
-            current_app.config['EXPIRE_CACHE_EIGHT_DAYS']
+            current_app.config['EXPIRE_CACHE_EIGHT_DAYS'],
+            raise_exception=True
         )

tests/app/commands/test_populate_redis.py

@@ -12,10 +12,12 @@ from tests.app.db import create_notification, create_template, create_service
 def test_populate_redis_template_usage_does_nothing_if_redis_disabled(mocker, notify_api, sample_service):
     mock_redis = mocker.patch('app.commands.redis_store')
     with set_config(notify_api, 'REDIS_ENABLED', False):
-        with pytest.raises(AssertionError):
+        with pytest.raises(SystemExit) as exit_signal:
             populate_redis_template_usage.callback.__wrapped__(sample_service.id, datetime.utcnow())
 
-    assert not mock_redis.called
+    assert mock_redis.mock_calls == []
+    # sys.exit with nonzero exit code
+    assert exit_signal.value.code != 0
 
 
 def test_populate_redis_template_usage_does_nothing_if_no_data(mocker, notify_api, sample_service):
@@ -23,7 +25,7 @@ def test_populate_redis_template_usage_does_nothing_if_no_data(mocker, notify_ap
     with set_config(notify_api, 'REDIS_ENABLED', True):
         populate_redis_template_usage.callback.__wrapped__(sample_service.id, datetime.utcnow())
 
-    assert not mock_redis.called
+    assert mock_redis.mock_calls == []
 
 
 @freeze_time('2017-06-12')
@@ -41,7 +43,8 @@ def test_populate_redis_template_usage_only_populates_for_today(mocker, notify_a
     mock_redis.set_hash_and_expire.assert_called_once_with(
         'service-{}-template-usage-2017-06-10'.format(sample_template.service_id),
         {str(sample_template.id): 3},
-        notify_api.config['EXPIRE_CACHE_EIGHT_DAYS']
+        notify_api.config['EXPIRE_CACHE_EIGHT_DAYS'],
+        raise_exception=True
     )
 
 
@@ -65,5 +68,6 @@ def test_populate_redis_template_usage_only_populates_for_given_service(mocker,
     mock_redis.set_hash_and_expire.assert_called_once_with(
         'service-{}-template-usage-2017-06-10'.format(s1.id),
         {str(t1.id): 2},
-        notify_api.config['EXPIRE_CACHE_EIGHT_DAYS']
+        notify_api.config['EXPIRE_CACHE_EIGHT_DAYS'],
+        raise_exception=True
     )