diff --git a/mpwo_api/mpwo_api/activities/activities.py b/mpwo_api/mpwo_api/activities/activities.py
index d91fc7d3..ff31b7c5 100644
--- a/mpwo_api/mpwo_api/activities/activities.py
+++ b/mpwo_api/mpwo_api/activities/activities.py
@@ -77,9 +77,7 @@ def get_activity_gpx(auth_user_id, activity_id):
         if not activity.gpx or activity.gpx == '':
             response_object = {
                 'status': 'fail',
-                'message': 'No gpx file for this activity (id: {})'.format(
-                    activity_id
-                )
+                'message': f'No gpx file for this activity (id: {activity_id})'
             }
             return jsonify(response_object), 400
 
@@ -103,7 +101,7 @@ def get_activity_gpx(auth_user_id, activity_id):
     else:
         gpx_content = ''
         status = 'not found'
-        message = 'Activity not found (id: {})'.format(activity_id)
+        message = f'Activity not found (id: {activity_id})'
         code = 404
 
     response_object = {
diff --git a/mpwo_api/mpwo_api/activities/models.py b/mpwo_api/mpwo_api/activities/models.py
index 49257e11..a037981b 100644
--- a/mpwo_api/mpwo_api/activities/models.py
+++ b/mpwo_api/mpwo_api/activities/models.py
@@ -86,7 +86,7 @@ class Sport(db.Model):
                               backref=db.backref('sports', lazy='joined'))
 
     def __repr__(self):
-        return '<Sport {!r}>'.format(self.label)
+        return f'<Sport {self.label!r}>'
 
     def __init__(self, label):
         self.label = label
@@ -149,8 +149,7 @@ class Activity(db.Model):
                                   single_parent=True))
 
     def __str__(self):
-        return '<Activity \'{}\' - {}>'.format(
-            self.sports.label, self.activity_date, )
+        return f'<Activity \'{self.sports.label}\' - {self.activity_date}>'
 
     def __init__(self, user_id, sport_id, activity_date, distance, duration):
         self.user_id = user_id
@@ -270,8 +269,8 @@ class ActivitySegment(db.Model):
     ave_speed = db.Column(db.Numeric(6, 2), nullable=True)   # km/h
 
     def __str__(self):
-        return '<Segment \'{}\' for activity \'{}\'>'.format(
-            self.segment_id, self.activity_id, )
+        return (f'<Segment \'{self.segment_id}\' '
+                f'for activity \'{self.activity_id}\'>')
 
     def __init__(self, segment_id, activity_id):
         self.segment_id = segment_id
@@ -319,11 +318,9 @@ class Record(db.Model):
     _value = db.Column("value", db.Integer, nullable=True)
 
     def __str__(self):
-        return '<Record {} - {} - {}>'.format(
-            self.sports.label,
-            self.record_type,
-            self.activity_date.strftime('%Y-%m-%d')
-        )
+        return (f'<Record {self.sports.label} - '
+                f'{self.record_type} - '
+                f"{self.activity_date.strftime('%Y-%m-%d')}>")
 
     def __init__(self, activity, record_type):
         self.user_id = activity.user_id
diff --git a/mpwo_api/mpwo_api/activities/utils.py b/mpwo_api/mpwo_api/activities/utils.py
index e79363c5..84b91799 100644
--- a/mpwo_api/mpwo_api/activities/utils.py
+++ b/mpwo_api/mpwo_api/activities/utils.py
@@ -47,8 +47,7 @@ def create_activity(
         new_activity.title = title
     else:
         sport = Sport.query.filter_by(id=new_activity.sport_id).first()
-        new_activity.title = '{} - {}'.format(sport.label,
-                                              new_activity.activity_date)
+        new_activity.title = f'{sport.label} - {new_activity.activity_date}'
 
     if gpx_data:
         new_activity.gpx = gpx_data['filename']
@@ -189,9 +188,9 @@ def get_file_path(auth_user_id, activity_file):
 
 def get_new_file_path(auth_user_id, activity_date, activity_file, sport):
     old_filename = secure_filename(activity_file.filename)
-    extension = '.{}'.format(old_filename.rsplit('.', 1)[1].lower())
+    extension = f".{old_filename.rsplit('.', 1)[1].lower()}"
     _, new_filename = tempfile.mkstemp(
-        prefix='{}_{}_'.format(activity_date, sport),
+        prefix=f'{activity_date}_sport_',
         suffix=extension
     )
     dir_path = os.path.join(
diff --git a/mpwo_api/mpwo_api/users/models.py b/mpwo_api/mpwo_api/users/models.py
index 31beecd6..aca9899c 100644
--- a/mpwo_api/mpwo_api/users/models.py
+++ b/mpwo_api/mpwo_api/users/models.py
@@ -27,7 +27,7 @@ class User(db.Model):
                               backref=db.backref('users', lazy='joined'))
 
     def __repr__(self):
-        return '<User %r>' % self.username
+        return f'<User {self.username!r}>'
 
     def __init__(
             self, username, email, password,