I am looking for python library to find out a key and tempo of the song recorded in MP3 format. I've found the music21 lib that allows doing that. But it seems like it works only with midi files. 
Does somebody know how to parse MP3 files using music21 and get the required sound characteristics? If it is impossible, please suggest another library.
Check out librosa. It can read mp3s and give some basic info such as tempo.
Is it possible to analyze mp3 file using music21?