'tests assigning a set to an empty set creates a nonempty
update statement and nonzero context size.'

def test_update_empty_set(self):

    c = SetUpdateClause(field='s', value=set())
    c._analyze()
    c.set_context_id(0)
    self.assertEqual(c._assignments, set())
    self.assertIsNone(c._additions)
    self.assertIsNone(c._removals)
    self.assertEqual(c.get_context_size(), 1)
    self.assertEqual(str(c), '"s"    =    %(0)s')
    ctx = {}
    c.update_context(ctx)
    self.assertEqual(ctx, {'0': set()})
