def test_removal_of_non_existent_node_should_fail():
g = Graph()
g.add(10)
assert set(g.nodes()) == {10}
try:
g.remove(12)
except KeyError:
assert True
else:
assert False
def test_can_remove():
g = Graph()
g.add(10)
assert set(g.nodes()) == {10}
g.remove(10)
assert set(g.nodes()) == set()
