怎么把numpy array转成scipy里的稀疏矩阵？

>>> import numpy as np
>>> from scipy import sparse
>>> arr = np.array([[0,2,0],[0,0,3],[1,0,0],[0,0,4]])
# numpy array转成稀疏
>>> arr_sparse = sparse.csr_matrix(arr) 
# 稀疏矩阵转成numpy array
>>> arr_numpy = arr_sparse.A