Returning boolean if set is empty

not as pythonic as the other answers, but mathematics:

return len(c) == 0

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As some comments wondered about the impact len(set) could have on complexity. It is O(1) as shown in the source code given it relies on a variable that tracks the usage of the set.

static Py_ssize_t
set_len(PyObject *so)
{
    return ((PySetObject *)so)->used;
}

def myfunc(a,b):
    c = a.intersection(b)
    return bool(c)

bool() will do something similar to not not, but more ideomatic and clear.