code: def longest_existing_path(_path):
      r
      existing_path = _path
      while True:
          _path_new = os.path.dirname(existing_path)
          if exists(_path_new):
              existing_path = _path_new
              break
          if _path_new == existing_path:
              print('!!! [utool] This is a very illformated path indeed.')
              existing_path = ''
              break
          existing_path = _path_new
      return existing_path
docs: r"""
      Returns the longest root of _path that exists
  
      Args:
          _path (str):  path string
  
      Returns:
          str: _path -  path string
  
      CommandLine:
          python -m utool.util_path --exec-longest_existing_path
  
      Example:
          >>> # ENABLE_DOCTEST
          >>> from utool.util_path import *  # NOQA
          >>> import utool as ut
          >>> target = dirname(ut.__file__)
          >>> _path = join(target, 'nonexist/foobar')
          >>> existing_path = longest_existing_path(_path)
          >>> result = ('existing_path = %s' % (str(existing_path),))
          >>> print(result)
          >>> assert existing_path == target