why the len(dendo) - 1?

[yaya159456]

>>> G=nx.erdos_renyi_graph(100, 0.01)
>>> dendo = generate_dendrogram(G)
>>> for level in range(len(dendo) - 1) :
>>>     print("partition at level", level,
>>>           "is", partition_at_level(dendo, level))

why need to set len(dendo) - 1? Other words,what the dendo[-1] means?