title: IOError when trying to open existing files
body: <p>I have a small issue with a python program that I wrote to extract some information from a special text file. The loop (code below) needs to execute my function <code>extract_zcoords()</code> over 500 files (1 file gives one list) so that I can build a dataset. </p>
  
  <pre><code>import os
  
  def extract_zcoord(filename): 
      f = open(filename, 'r')         
      ... # do something with f
  
  
  ### LOOP OVER DIRECTORY 
  location = '/Users/spyros/Desktop/3NY8MODELSHUMAN/HomologyModels' 
  
  for filename in os.listdir(location):
      extract_zcoord(filename) 
  </code></pre>
  
  <hr>
  
  <p>THE ERROR: 
  The IOException <code>No such file or directory</code> is the one that occurs, so for some reason python is not accessing the files. I have checked directory pathname (location) and file permissions, and they are correct (read+write). Any ideas why an IOError would be reported when the files do exist and pathname is correct?</p>
  
  <p>Any ideas what might be wrong?   </p>