code: def add_cmd(argv):
      
      parser = argparse.ArgumentParser()
      parser.add_argument('-d', dest='remove', action='store_true')
      parser.add_argument('dirs', nargs='+')
      args = parser.parse_args(argv)
  
      extra_paths = sitepackages_dir() / '_virtualenv_path_extensions.pth'
      new_paths = [os.path.abspath(d) + "\n" for d in args.dirs]
      if not extra_paths.exists():
          with extra_paths.open('w') as extra:
              extra.write('''import sys; sys.__plen = len(sys.path)
  import sys; new=sys.path[sys.__plen:]; del sys.path[sys.__plen:]; p=getattr(sys,'__egginsert',0); sys.path[p:p]=new; sys.__egginsert = p+len(new)
              ''')
  
      def rewrite(f):
          with extra_paths.open('r+') as extra:
              to_write = f(extra.readlines())
              extra.seek(0)
              extra.truncate()
              extra.writelines(to_write)
  
      if args.remove:
          rewrite(lambda ls: [line for line in ls if line not in new_paths])
      else:
          rewrite(lambda lines: lines[0:1] + new_paths + lines[1:])
docs: Add the specified directories to the Python path for the currently active virtualenv.
  
  This will be done by placing the directory names in a path file named
  "virtualenv_path_extensions.pth" inside the virtualenv's site-packages
  directory; if this file does not exists, it will be created first.