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Марк, os.open added dir_fd support in 3.3, which is implemented on POSIX systems by calling openat. The dir_fd parameter is available for many os functions. This is discussed in section 1.5, Files and Directories [1].
It would be nice if we could support dir_fd on Windows as well, but we'd have to bypass the CRT and Windows API to use the native NT API instead, such as NtCreateFile [2]. The kernel has supported opening a file relative to a directory handle since it was release in 1993 (NT 3.1). All named kernel objects are referenced using an OBJECT_ATTRIBUTES [3] data structure. ObjectName -- a path with up to 32768 UTF-16 characters -- is relative to the RootDirectory handle if non-NULL. This is how paths relative to the process working directory are implemented, but changing the working directory isn't thread safe. 
[1]: https://docs.python.org/3/library/os.html#files-and-directories
[2]: https://msdn.microsoft.com/en-us/library/ff566424
[3]: https://msdn.microsoft.com/en-us/library/ff557749