How to find the graphs from the following collection whose determinant is maximum?

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If you want a single graph, there is a one-liner:

g = max(graphs.nauty_geng("-c 7 9:9"), key=lambda g: abs(g.adjacency_matrix().determinant()))

Along these lines, we can make two passes over the graphs to get all graphs with the maximum determinant:

max_det = max(abs(g.adjacency_matrix().determinant()) for g in graphs.nauty_geng("-c 7 9:9"))
G = filter(lambda g: abs(g.adjacency_matrix().determinant())==max_det, graphs.nauty_geng("-c 7 9:9"))

Finally, if only one pass is preferred, it can be done like this:

G = []
max_det = -oo
for g in graphs.nauty_geng("-c 7 9:9"):
    A = abs(g.adjacency_matrix().determinant())
    if A > max_det:
        max_det = A
        G = [g]
    elif A == max_det:
        G.append(g)