Counting removed items in a Set in Python

Given开发者_JS百科 two sets

a = [5,3,4,1,2,6,7]
b = [1,2,4,9]
c = set(a) - set(b)
# c -> [5,3,6,7]

is it possible to count how many items were removed from set 'a' ?

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How about len(set(a)) - len(c)?

Edit: len(a) could be incorrect if a contains duplicates.

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there might be a more efficient way, but

 len(set(a)-set(c))

will work

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Assuming lack of duplicates: len(a)-len(c) otherwise try: len(set(a)) - len(c)

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a = [5,3,4,1,2,6,7] 
b = [1,2,4,9] 
c = set(a) - set(b)

print len(c)