python setdefault in indexing text file

indx={}
with open(filename) as f:
    for n,line in enumerate(f):
        for word in line.split():
         indx.setdefault(word,[]).append(n)
#emit in alphabetical order 
for word in sorted(indx):
    print "%s:" % word
    for n in indx[word]: print n,
    print

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