def test_tables():
md = MetaData()
assert md._tables == {}
md.add(DummyMapper('a'))
md.add(DummyMapper('b'))
md.add(DummyMapper('c'))
md.add(DummyMapper('d'))
with pytest.raises(InvalidRequestError):
md.add(DummyMapper('a'))
assert DummyMapper('a') in md
md.remove(DummyMapper('a'))
assert DummyMapper('a') not in md
assert set(md) == set(['b', 'c', 'd'])
assert len(list(md.values())) == 3
assert len(list(md.items())) == 3
md.clear()
assert len(list(md)) == 0
