if key not in d:
    d[key] = 0
d[key] += count

Dictionary of words mapped to how many times the word appears in the file
Dictionary is called d1

For example d1["and"] = 8, d1["or"] = 8, d1["cat"]= 3

Invert dictionary to d2

d2[8] = ["and", "or"]
d2[3] = ["cat"]

to create d2

for (word,cnt) in d1.items():
    if cnt not in d2:
       d2[cnt] = [word]  # create list , first time
    else: # cnt is already in d2
       d2[cnt].append(word)


   if key not in d3:
       d3[key] = set(value)
   else:
       d3[key].add(value)     



a = set([])
print a
set([])

b = set()
print b
set([])


print a
set([])

x = [ 4, 6, 1, 2, 7, 4, 6]
y = [a.add(w)  for w in x]
print y
[None, None, None, None, None, None, None]

print a
set([1, 2, 4, 6, 7])

z = set([w for w in x])
print z
set([1, 2, 4, 6, 7])