def test_zfill(self, size):
        SUBSTR = '-568324723598234'
        s = SUBSTR.zfill(size)
        self.failUnless(s.endswith('0' + SUBSTR[1:]))
        self.failUnless(s.startswith('-0'))
        self.assertEquals(len(s), size)
        self.assertEquals(s.count('0'), size - len(SUBSTR))