title: Scipy -- 3d griddata -- Why is it necessary to cast griddata xi argument to tuple?
body: <p>Why does the below call to griddata fail? </p>
  
  <pre><code>import scipy.interpolate
  import numpy as np
  grid_vals = np.meshgrid(*([np.linspace(-1,1,200)] * 3))
  interp_vals = scipy.interpolate.griddata(np.random.randn(50,3), np.random.randn(50), grid_vals, 'linear')
  </code></pre>
  
  <p>The following exception occurs:
  ValueError: number of dimensions in xi does not match x</p>
  
  <p>If i cast the xi (grid_vals) argument to tuple: </p>
  
  <pre><code>interp_vals = scipy.interpolate.griddata(np.random.randn(50,3), np.random.randn(50), tuple(grid_vals), 'linear') 
  </code></pre>
  
  <p>The error goes away. Why? </p>