title: AttributeError: 'numpy.ndarray' object has no attribute 'step'
body: <p>I want to get 3d grid and I use the following command:</p>
  
  <pre>
  
      x = np.linspace(-10,10,100)
      y = np.linspace(-10,10,100)
      z = np.linspace(-10,10,100)
  
      X,Y,Z = np.mgrid[x, y, z]
  
  </pre>
  
  <p>And such error emerges</p>
  
  <pre>
  
          AttributeError                            Traceback (most recent call last)
       in ()
            6 
            7 
      ----> 8 X,Y,Z = np.mgrid[x, y, z]
            9 
           10 #X,Y,Z = np.mgrid[-10:10, -10:10, -10:10]
  
      ~/Desktop/PyProjects/Phys/env/lib/python3.6/site-packages/numpy/lib/index_tricks.py in __getitem__(self, key)
          165             typ = int
          166             for k in range(len(key)):
      --> 167                 step = key[k].step
          168                 start = key[k].start
          169                 if start is None:
  
      AttributeError: 'numpy.ndarray' object has no attribute 'step'
  
  </pre>
  
  <p>I know I can use this form to generate the grid</p>
  
  <pre>
  
      X,Y,Z = np.mgrid[-10:10, -10:10, -10:10]
  
  </pre>
  
  <p>but it is not mobile for me</p>