Thanks for the patch, it looks correct. The Frm class takes a variable number of *args, and then returns “format % args”. It should at least be applied to the 2.7 branch.

For 3.4+, perhaps we can eliminate the need for the Frm class altogether, by using TestCase.subTest():

def check_bitop_identities_1(self, x):
    eq = self.assertEqual
    with self.subTest(x=x):
        eq(x & 0, 0)
        ...
        for n in range(2*SHIFT):
            p2 = 2 ** n
            with self.subTest(n=n, p2=p2):
                eq(x << n >> n, x)
                ...
                eq(x & -p2, x & ~(p2 - 1))