def test_arbitrary_node():
# given
g = UndirectedGraph()
g.add_edge(1, 2)
g.add_edge(2, 3)
u = g.arbitrary_node()
assert u in g.nodes
def test_arbitrary_node_raises_if_graph_empty():
# given
g = UndirectedGraph()
with pytest.raises(DoesNotExistError):
u = g.arbitrary_node()