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With the example you started with:

K = SimplicialComplex([[0, 1], [1, 2], [2, 0]])
C1 = K.n_chains(1)

Then C1 is

Free module generated by {(0, 1), (0, 2), (1, 2)} over Integer Ring

so we can do

c01, c02, c12 = C1.gens()
z = c01 - c02 + c12

Then

z.is_cycle()

should return True, while

z.is_boundary()

should return False. So z represents a nonzero class in homology.