for m in bpy.context.object.material_slots:
mat = m.material
nodes = mat.node_tree.nodes
for mat_node in mat.node_tree.nodes:
    if mat_node.type == 'BSDF_PRINCIPLED':
        print(mat.name + " "+ mat_node.name +" :" )
        alphasocket = mat_node.inputs[18]
        #print(alphasocket)
        node_in = mat_node.inputs
        #print(node_in)

        for n in alphasocket.links:
            print(n.from_node.name +" connected to Alpha")