title: Reading data blocks from a file in Python
body: <p>I'm new to python and am trying to read "blocks" of data from a file. The file is written something like:</p>
  
  <pre><code># Some comment
  # 4 cols of data --x,vx,vy,vz
  # nsp, nskip =           2          10
  
  
  #            0   0.0000000
  
  
  #            1           4
   0.5056E+03  0.8687E-03 -0.1202E-02  0.4652E-02
   0.3776E+03  0.8687E-03  0.1975E-04  0.9741E-03
   0.2496E+03  0.8687E-03  0.7894E-04  0.8334E-03
   0.1216E+03  0.8687E-03  0.1439E-03  0.6816E-03
  
  
  #            2           4
   0.5056E+03  0.8687E-03 -0.1202E-02  0.4652E-02
   0.3776E+03  0.8687E-03  0.1975E-04  0.9741E-03
   0.2496E+03  0.8687E-03  0.7894E-04  0.8334E-03
   0.1216E+03  0.8687E-03  0.1439E-03  0.6816E-03
  
  
  #          500  0.99999422
  
  
  #            1           4
   0.5057E+03  0.7392E-03 -0.6891E-03  0.4700E-02
   0.3777E+03  0.9129E-03  0.2653E-04  0.9641E-03
   0.2497E+03  0.9131E-03  0.7970E-04  0.8173E-03
   0.1217E+03  0.9131E-03  0.1378E-03  0.6586E-03
  
  and so on
  </code></pre>
  
  <p>Now I want to be able specify and read only one block of data out of these many blocks. I'm using <code>numpy.loadtxt('filename',comments='#')</code> to read the data but it loads the whole file in one go. I searched online and someone has created a patch for the numpy io routine to specify reading blocks but it's not in mainstream numpy. </p>
  
  <p>It's much easier to choose blocks of data in gnuplot but I'd have to write the routine to plot the distribution functions. If I can figure out reading specific blocks, it would be much easier in python. Also, I'm moving all my visualization codes to python from IDL and gnuplot, so it'll be nice to have everything in python instead of having things scattered around in multiple packages.</p>
  
  <p>I thought about calling gnuplot from within python, plotting a block to a table and assigning the output to some array in python. But I'm still starting and I could not figure out the syntax to do it.</p>
  
  <p>Any ideas, pointers to solve this problem would be of great help.</p>