Difference of two sets of tuples in Python

Question

I have two sets of tuples a and b .

a = set([(1,'a'),(34,'b'), (82,'c')])
b = set([(8,'a'),(98,'c')])

I want a set c such that

c = set([(34,'b')])

What is the most efficient way of doing this operation in python ?

Answer 1

I guess O(n) is the most efficient:

>>> b2 = {x for n,x in b}
>>> c = {(n,x) for (n,x) in a if x not in b2}
>>> c
{(34, 'b')}

Answer 2

I would start by using dicts instead of sets for a and b:

a = {k: v for (v, k) in a}
b = {k: v for (v, k) in b}

Then, it is just a matter of taking the content of a that is not in b

print [(k, a[k]) for k in set(a.keys()) - set(b.keys())]

or

b_keys = set(b.keys())
print [(k,v) for (k,v) in a.items() if k not in b_keys}