title: How to deal with this AttributeError in scipy? It's very strange
body: <p>This code is just from <a href="https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.save_npz.html#scipy.sparse.save_npz" rel="nofollow noreferrer">SciPy.org</a>:</p>
  <pre><code>import numpy as np
  import scipy.sparse
  
  sparse_matrix = scipy.sparse.csc_matrix(np.array([[0, 0, 3], [4, 0, 0]]))
  scipy.sparse.save_npz('/tmp/sparse_matrix.npz', sparse_matrix)
  </code></pre>
  <p>But when I run it, it shows: AttributeError: module 'scipy.sparse' has no attribute 'save_npz'</p>
  <p>And if I do this:</p>
  <pre><code>import numpy as np
  import scipy.sparse.save_npz
  import scipy.sparse
  
  sparse_matrix = scipy.sparse.csc_matrix(np.array([[0, 0, 3], [4, 0, 0]]))
  scipy.sparse.save_npz('/tmp/sparse_matrix.npz', sparse_matrix)
  </code></pre>
  <p>It shows: ModuleNotFoundError: No module named 'scipy.sparse.save_npz'</p>
  <p>And if I do this:</p>
  <pre><code>import numpy as np
  from scipy.sparse import csr_matrix, save_npz
  
  sparse_matrix = csc_matrix(np.array([[0, 0, 3], [4, 0, 0]]))
  save_npz('/tmp/sparse_matrix.npz', sparse_matrix)
  </code></pre>
  <p>It shows: ImportError: cannot import name 'save_npz'</p>
  <p>So, how to fix it?</p>