They allow users to discover new songs or artists they may like within large music catalogs, and they are known to improve the overall user experience on these services [5, 22]. In particular, the French music streaming service Deezer [7], offering 90 million music tracks to 16 million active users from 180 countries, extensively relies on its homemade Flow feature to recommend music. Flow materializes as a simple button, proposed to Deezer users on the homepage of the service. A click on this button launches a personalized and virtually infinite radio-style playlist of songs, computed internally using collaborative filtering methods [3, 16]. However, despite promising results over the past years, Flow used to ignore the moods of users when generating playlists.