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]])
原文：http://www.cnblogs.com/liziran/p/6711229.html