<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_vertices_neighbours(nets):
 """Generate a listing of each vertex's immedate neighbours in an undirected interpretation of a graph. Returns ------- """

    zero_fn = (lambda: 0)
    vertices_neighbours = defaultdict(lambda: defaultdict(zero_fn))
    for net in nets:
        if net.weight != 0:
            for sink in net.sinks:
                vertices_neighbours[net.source][sink] += net.weight
                vertices_neighbours[sink][net.source] += net.weight
    return vertices_neighbours