title: Fill in missing values with nearest neighbour in Python numpy masked arrays?
body: <p>I am working with a 2D Numpy masked_array in Python.
  I need to change the data values in the masked area such that they equal the nearest unmasked value.</p>
  
  <p>NB. If there are more than one nearest unmasked values then it can take any of those nearest values (which ever one turns out to be easiest to code…)</p>
  
  <p>e.g.</p>
  
  <pre><code>import numpy
  import numpy.ma as ma
  
  a = numpy.arange(100).reshape(10,10)
  fill_value=-99
  a[2:4,3:8] = fill_value
  a[8,8] = fill_value
  a = ma.masked_array(a,a==fill_value)
  
  &gt;&gt;&gt; a  [[0 1 2 3 4 5 6 7 8 9]
    [10 11 12 13 14 15 16 17 18 19]
    [20 21 22 -- -- -- -- -- 28 29]
    [30 31 32 -- -- -- -- -- 38 39]
    [40 41 42 43 44 45 46 47 48 49]
    [50 51 52 53 54 55 56 57 58 59]
    [60 61 62 63 64 65 66 67 68 69]
    [70 71 72 73 74 75 76 77 78 79]
    [80 81 82 83 84 85 86 87 -- 89]
    [90 91 92 93 94 95 96 97 98 99]],
  </code></pre>
  
  <ul>
  <li>I need it to look like this:</li>
  </ul>
  
  <blockquote>
  <pre><code>&gt;&gt;&gt; a.data
   [[0 1 2 3 4 5 6 7 8 9]
   [10 11 12 13 14 15 16 17 18 19]
   [20 21 22 ? 14 15 16 ? 28 29]
   [30 31 32 ? 44 45 46 ? 38 39]
   [40 41 42 43 44 45 46 47 48 49]
   [50 51 52 53 54 55 56 57 58 59]
   [60 61 62 63 64 65 66 67 68 69]
   [70 71 72 73 74 75 76 77 78 79]
   [80 81 82 83 84 85 86 87 ? 89]
   [90 91 92 93 94 95 96 97 98 99]],
  </code></pre>
  </blockquote>
  
  <p>NB. where "?" could take any of the adjacent unmasked values.</p>
  
  <p>What is the most efficient way to do this?</p>
  
  <p>Thanks for your help.</p>