docs: def add_unique_postfix(fn):
      """__source__ = 'http://code.activestate.com/recipes/577200-make-unique-file-name/'"""
      if not os.path.exists(fn):
          return fn
  
      path, name = os.path.split(fn)
      name, ext = os.path.splitext(name)
  
      make_fn = lambda i: os.path.join(path, '%s(%d)%s' % (name, i, ext))
  
      for i in xrange(2, sys.maxint):
          uni_fn = make_fn(i)
          if not os.path.exists(uni_fn):
              return uni_fn