code: def _find_file(filename):
      
      import os
      if os.path.exists(filename):
          path = filename
      else:
          path = os.path.dirname(__file__)
          path = os.path.join(path, 'data', filename)
  
      if not os.path.exists(path):
          raise ValueError("cannot locate file '%s'" %filename)
  
      return path
docs: Find the file path, first checking if it exists and then looking in the
      data directory