What you're after is the ID3 module. It's very simple and will give you exactly what you need. Just copy the ID3.py file into your site-packages directory and you'll be able to do something like the following:
from ID3 import *
try:
id3info = ID3('file.mp3')
print id3info
# Change the tags
id3info['TITLE'] = "Green Eggs and Ham"
id3info['ARTIST'] = "Dr. Seuss"
for k, v in id3info.items():
print k, ":", v
except InvalidTagError, message:
print "Invalid ID3 tag:", message