def test_missing_key():
"""
If an unindexed key pair is passed, return None.
"""
m = Matrix()
m.set_pair('a', 'b', 1)
assert m.get_pair('a', 'c') == None
def test_set_pair():
"""
set_pair() should set the value under an order-independent key.
"""
m = Matrix()
m.set_pair('a', 'b', 1)
assert m.get_pair('a', 'b') == 1
assert m.get_pair('b', 'a') == 1
def test_update_key_set():
"""
Keys should be added to a set of stored keys.
"""
m = Matrix()
m.set_pair('a', 'b', 1)
m.set_pair('a', 'c', 2)
assert m.keys == set(['a', 'b', 'c'])