def test_add_edge_neighbors():
'''
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 = UndirectedGraph()
# Edges can connect existing nodes
assert g.add_node('a')
assert g.add_node('b')
assert g.add_edge('a', 'b') # Default weight is 1
assert ('b', 1) in g.neighbors('a')
assert ('a', 1) in g.neighbors('b')
# Adding an edge that already exists should be rejected, and return False
assert g.add_edge('a', 'b', 2) == False
assert ('b', 1) in g.neighbors('a')
assert ('a', 1) in g.neighbors('b')
# Adding an edge with a non-existing node works just fine
assert g.add_edge('b', 'c', 2)
assert ('b', 1) in g.neighbors('a')
assert ('a', 1) in g.neighbors('b')
assert ('c', 2) in g.neighbors('b')
assert ('b', 2) in g.neighbors('c')
def test_simple_integration():
'''
test a simple use case, retesting assumptions at each step.
'''
g = UndirectedGraph()
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) == 2
assert ('a', 'b', 1) in edges
assert ('b', 'a', 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'))) == 1
assert ('a', 1) in g.neighbors('b')
assert len(list(g.neighbors('c'))) == 0
g.add_edge('a', 'c')
edges = list(g.edges())
assert len(edges) == 4
assert ('a', 'b', 1) in edges
assert ('b', 'a', 1) in edges
assert ('a', 'c', 1) in edges
assert ('c', 'a', 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'))) == 1
assert ('a', 1) in g.neighbors('b')
assert len(list(g.neighbors('c'))) == 1
assert ('a', 1) in g.neighbors('c')
edges = list(g.edges())
assert ('a', 'b', 1) in edges
assert ('b', 'a', 1) in edges
assert ('a', 'c', 1) in edges
assert ('c', 'a', 1) in edges
