index = dict.serdefault(key,default)
尝试往dict中插入新键值key,如果key已存在就原dict不变,否则插入key:defalut;返回值为key在dict中的下标
可以用来实现稀疏矩阵https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.csr_matrix.html
>>> docs = [["hello", "world", "hello"], ["goodbye", "cruel", "world"]]
>>> indptr = [0]
>>> indices = []
>>> data = []
>>> vocabulary = {}
>>> for d in docs:
... for term in d:
... index = vocabulary.setdefault(term, len(vocabulary))
... indices.append(index)
... data.append(1)
... indptr.append(len(indices))
...
>>> csr_matrix((data, indices, indptr), dtype=int).toarray()
array([[2, 1, 0, 0],
[0, 1, 1, 1]])