def test_add_edge_nodes():
'''
Adding an edge between non-existant edges should cause those nodes to be
returned by .nodes().
'''
g = DirectedGraph()
assert g.add_edge('a', 'b', 42)
assert len(list(g.nodes())) == 2
assert 'a' in g.nodes()
assert 'b' in g.nodes()
def test_add_nodes():
'''
Adding a node should cause that node to appear in the return value of .nodes().
'''
g = DirectedGraph()
assert g.add_node('a')
assert 'a' in g.nodes()
assert len(list(g.nodes())) == 1
assert g.add_node('a') == False
assert 'a' in g.nodes()
assert len(list(g.nodes())) == 1
assert g.add_node('b')
assert 'a' in g.nodes()
assert 'b' in g.nodes()
assert len(list(g.nodes())) == 2
def test_simple_integration():
'''
test a simple use case, retesting assumptions at each step.
'''
g = DirectedGraph()
assert g.add_node('a')
assert g.contains('a')
assert len(list(g.nodes())) == 1
assert 'a' in g.nodes()
assert len(list(g.neighbors('a'))) == 0
assert g.add_node('b')
assert g.contains('b')
assert len(list(g.nodes())) == 2
assert 'a' in g.nodes()
assert 'b' in g.nodes()
assert len(list(g.neighbors('a'))) == 0
assert len(list(g.neighbors('b'))) == 0
assert g.add_node('c')
assert g.contains('c')
assert len(list(g.nodes())) == 3
assert 'a' in g.nodes()
assert 'b' in g.nodes()
assert 'c' in g.nodes()
assert len(list(g.neighbors('a'))) == 0
assert len(list(g.neighbors('b'))) == 0
assert len(list(g.neighbors('c'))) == 0
g.add_edge('a', 'b')
edges = list(g.edges())
assert len(edges) == 1
assert ('a', 'b', 1) in edges
assert g.contains('a')
assert g.contains('b')
assert g.contains('c')
assert len(list(g.nodes())) == 3
assert 'a' in g.nodes()
assert 'b' in g.nodes()
assert 'c' in g.nodes()
assert len(list(g.neighbors('a'))) == 1
assert ('b', 1) in g.neighbors('a')
assert len(list(g.neighbors('b'))) == 0
assert len(list(g.neighbors('c'))) == 0
g.add_edge('a', 'c')
edges = list(g.edges())
assert len(edges) == 2
assert ('a', 'b', 1) in edges
assert ('a', 'c', 1) in edges
assert g.contains('a')
assert g.contains('b')
assert g.contains('c')
assert len(list(g.nodes())) == 3
assert 'a' in g.nodes()
assert 'b' in g.nodes()
assert 'c' in g.nodes()
assert len(list(g.neighbors('a'))) == 2
assert ('b', 1) in g.neighbors('a')
assert ('c', 1) in g.neighbors('a')
assert len(list(g.neighbors('b'))) == 0
assert len(list(g.neighbors('c'))) == 0
edges = list(g.edges())
assert ('a', 'b', 1) in edges
assert ('a', 'c', 1) in edges