def test_add_regular_edges(self):
# Adds an edge to edge-less graph
v, w, e = self.v, self.w, self.e
g = Graph([v, w], [])
g.add_regular_edges(1)
eq_(g.edges(), [e])
# Can make a regular graph of three vertices and degree 2
x = Vertex('x')
e2 = Edge(v, x)
e3 = Edge(w, x)
g = Graph([v, w, x], [])
g.add_regular_edges(2)
eq_(g.edges(), [e, e2, e3])
# Cannot make a regular graph of three vertices and degree 1
g = Graph([v, w, x], [])
assert_raises(GraphError, g.add_regular_edges, 1)
# Cannot make a regular graph of three vertices and degree 3
g = Graph([v, w, x], [])
assert_raises(GraphError, g.add_regular_edges, 3)
# Can make a regular graph of five vertices and even degree
y = Vertex('y')
z = Vertex('z')
g = Graph([v, w, x, y, z], [])
g.add_regular_edges(2)
eq_(g.num_edges(), 5)
# Can make a regular graph of 4 vertices and odd degree
g = Graph([v, w, x, y], [])
g.add_regular_edges(3)
eq_(g.num_edges(), 6)
# Cannot make a regular graph of five vertices and degree 3
g = Graph([v, w, x, y, z], [])
assert_raises(GraphError, g.add_regular_edges, 3)
