diff.py

import difflib

# initialize
matcher = difflib.SequenceMatcher()
ret = ['this', 'is', 'sparta']
rel = ['this', 'here', 'is','not','acutally', 'sparta']
    
# calculate matching blocks    
matcher.set_seqs(ret, rel)
matches = matcher.get_matching_blocks()[:-1]

# get_matching_blocks returns a list of triples (named tuples)
# each triple (i, j, n) denoting that ret[i:i+n] == rel[j:j+n]
# the last 'dummy' triple is discarded since it's equal to (len(rel), len(ret), 0)
rel_intersect_ret = sum(i.size for i in matches)

print rel_intersect_ret