def test_add_edge_edges():
'''
add_edge should connect existing nodes in the graph, causing neighbors to
return the connected edges.
add_edge should not overwrite existing edges and return False if the user tries.
add_edge should create nodes if they do not already exist.
'''
g = DirectedGraph()
# Edges can connect existing nodes
assert g.add_node('a')
assert g.add_node('b')
assert g.add_edge('a', 'b')
assert len(list(g.edges())) == 1
assert ('a', 'b', 1) in g.edges()
# Adding an edge that already exists should be rejected, and return False
assert g.add_edge('a', 'b', 2) == False
assert len(list(g.edges())) == 1
assert ('a', 'b', 1) in g.edges()
# Adding an edge with a non-existing node works just fine
assert g.add_edge('b', 'c', 2)
assert len(list(g.edges())) == 2
assert ('a', 'b', 1) in g.edges()
assert ('b', 'c', 2) in g.edges()
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