title: Finding missing values in a numpy array
body: <p>Alright, extreme rookie question here.  In my program, I generate a 2D numpy array, some of whom's entries are missing (not the "nan" kind of nonexistant, but the "None" kind, or NoneType).  I'd like to put a mask over these entries, but I seem to be having some trouble doing so.  Ordinarily, to mask over, say, all entries with value 2, I'd do</p>
  
  <p>A = np.ma.masked_where(A[A==2], A)</p>
  
  <p>In this case, that doesn't seem to work no matter what I try for the first parameter.  Thoughts?</p>