# Natural Language Toolkit: compound_keys

attachment = nltk.defaultdict(lambda:[0,0])
V, N = 0, 1
for entry in nltk.corpus.ppattach.attachments('training'):
    key = entry.verb, entry.prep
    if entry.attachment == 'V':
        attachment[key][V] += 1
    else:
        attachment[key][N] += 1