In[1]: x2
Out[1]: array([[12,  5,  2,  4],
                [ 7,  6,  8,  8],
                [ 1,  6,  7,  7]])

In[2]: x2[:2, :3]  # 两行，三列
Out[2]: array([[12,  5,  2], 
               [ 7,  6,  8]])
In[3]: x2[:3, ::2]  # 所有行，每隔一列
Out[3]: array([[12,  2],
                [ 7,  8],
                [ 1,  7]])

In[4]: x2[::-1, ::-1]
Out[4]: array([[ 7,  7,  6,  1],
                [ 8,  8,  6,  7],
                [ 4,  2,  5,  12]])