Since many of os functions were polymorphic in its first argument (path or fd), the logical continuation is to add a (path, dir_fd) pair as yet one case of this argument. In any case dir_fd is incopatible with fd. This simplifies the code of certain functions which takes patch and dir_fd and pass them to lower level without changes, or that use same patch and dir_fd in calls of several functions. This freely adds support of dir_fd to many functions, such as os.path.isdir. This solves the problem of naming multiple dir_fd arguments (src_dir_fd/dst_dir_fd looks ugly). This saves a pre-3.3 signature of some functions, which will facilitate the use of decorators and improve compatibility with alternative implementations.