defining function for graph (help needed)

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I need help defining a function that does the following (I'm very new to Python/Sage).

I need a function called ihara(G) where G is any graph input by the user.

I need the function ihara(G) to return the Z resulting from all the calculations below:

D = G.to_directed()
L = D.line_graph()
L.delete_edges([((x,y,None), (y,x,None)) for x,y in G.edges( labels=None)])
L.delete_edges([((x,y,None), (y,x,None)) for y,x in G.edges( labels=None)])

IM = identity_matrix(QQ,D.size())
T = L.adjacency.matrix()

var('u')
X = IM-u*T
Z = X.det()

Could anybody help me with this? or at least get me started in the right direction...