diff --git a/zfs_backup.py b/zfs_backup.py
index ae85731..8554e9f 100755
--- a/zfs_backup.py
+++ b/zfs_backup.py
@@ -722,7 +722,9 @@ class Dataset:
         for interval, number in config.keep_snapshots_per_interval.items():
             self._clean_old_snapshots_for_interval(interval, number)
 
-    def _clean_old_snapshots_for_interval(self, interval: str, max_count: int) -> None:  # TODO I assume the regex matches across different datasets, which it shouldn't
+    def _clean_old_snapshots_for_interval(
+        self, interval: str, max_count: int
+    ) -> None:  # TODO I assume the regex matches across different datasets, which it shouldn't
         regex = _get_regex_matching_snapshots_with_tags([config.snapshot_tag, interval])
         snapshots = [
             snapshot for snapshot in self.snapshots if snapshot.matches_regex(regex)