Counting removed items in a Set in Python

Question

Given 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' ?

Answer 1

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

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

Answer 2

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

Answer 3

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

print len(c)

Answer 4

there might be a more efficient way, but

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

will work