diff --git a/luigi/contrib/mongodb.py b/luigi/contrib/mongodb.py
index 640d46652b..7e4a6069bf 100644
--- a/luigi/contrib/mongodb.py
+++ b/luigi/contrib/mongodb.py
@@ -36,7 +36,7 @@ def __init__(self, mongo_client, index, collection):
         self._collection = collection
 
     def __str__(self):
-        return '%s/%s' % (self._index, self._collection)
+        return f'{self._index}/{self._collection}'
 
     def get_collection(self):
         """
diff --git a/luigi/contrib/sqla.py b/luigi/contrib/sqla.py
index 7a64e85130..f292c21354 100644
--- a/luigi/contrib/sqla.py
+++ b/luigi/contrib/sqla.py
@@ -190,7 +190,7 @@ def __init__(self, connection_string, target_table, update_id, echo=False, conne
         self.marker_table_bound = None
 
     def __str__(self):
-        return self.marker_table_bound or self.connection_string
+        return self.target_table
 
     @property
     def engine(self):
diff --git a/luigi/worker.py b/luigi/worker.py
index c678487fa3..c3ea777b8a 100644
--- a/luigi/worker.py
+++ b/luigi/worker.py
@@ -187,7 +187,7 @@ def run(self):
                     if not self.check_complete(dep):
                         nonexistent_outputs = [output for output in dep.output() if not output.exists()]
                         if nonexistent_outputs:
-                            missing.append('%s (%s)' % (dep.task_id, ', '.join(map(str, nonexistent_outputs))))
+                            missing.append(f'{dep.task_id} ({", ".join(map(str, nonexistent_outputs))})')
                         else:
                             missing.append(dep.task_id)
                 if missing: