diff --git a/app/playlists.py b/app/playlists.py
index df02ae9..98c37db 100644
--- a/app/playlists.py
+++ b/app/playlists.py
@@ -457,7 +457,13 @@ class Playlist(QTableWidget):
         )
 
     def _mark_next_track(self):
-        "Set up metadata for next track in playlist if there is one."
+        """
+        Find next track to play.
+
+        If not found, return None.
+
+        If found, mark row with metadata and return track_id.
+        """
 
         found_next_track = False
         current_row = self._meta_get_current()
diff --git a/requirements.txt b/requirements.txt
index 2c90e8b..6a836a3 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,30 +1,39 @@
 alembic==1.5.7
 backcall==0.2.0
 decorator==5.0.7
+flake8==3.9.1
 greenlet==1.0.0
 importlib-metadata==3.7.3
+ipdb==0.13.7
 ipython==7.22.0
 ipython-genutils==0.2.0
 jedi==0.18.0
 Mako==1.1.4
 MarkupSafe==1.1.1
+mccabe==0.6.1
 mysqlclient==2.0.3
 parso==0.8.2
 pexpect==4.8.0
 pickleshare==0.7.5
+pkg-resources==0.0.0
 prompt-toolkit==3.0.18
 ptyprocess==0.7.0
+pycodestyle==2.7.0
 pydub==0.25.1
+pyflakes==2.3.1
 Pygments==2.8.1
 PyQt5==5.15.4
 PyQt5-Qt5==5.15.2
 PyQt5-sip==12.8.1
 python-dateutil==2.8.1
 python-editor==1.0.4
+python-slugify==5.0.0
 python-vlc==3.0.11115
 six==1.15.0
 SQLAlchemy==1.4.2
+text-unidecode==1.3
 tinytag==1.5.0
+toml==0.10.2
 traitlets==5.0.5
 typing-extensions==3.7.4.3
 wcwidth==0.2.5