f-strings everywhere

docs/tutorial/blog.rst

@@ -207,7 +207,7 @@ it from each view.
         ).fetchone()
 
         if post is None:
-            abort(404, "Post id {0} doesn't exist.".format(id))
+            abort(404, f"Post id {id} doesn't exist.")
 
         if check_author and post['author_id'] != g.user['id']:
             abort(403)

docs/tutorial/views.rst

@@ -94,7 +94,7 @@ write templates to generate the HTML form.
             elif db.execute(
                 'SELECT id FROM user WHERE username = ?', (username,)
             ).fetchone() is not None:
-                error = 'User {} is already registered.'.format(username)
+                error = f"User {username} is already registered."
 
             if error is None:
                 db.execute(