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In Python 3.3 (but not earlier) os.stat() is documented to work with file descriptors. (os.path.exists() also works with fds since it is implemented in terms of os.stat(), although that is *not* documented.)
However, on Windows if fd is not open then os.stat(fd) triggers an assertion error or crash:
 File: f:\dd\vctools\crt_bld\self_x86\crt\src\osfinfo.c
 Line: 316
 Expression: (_osfile(fh) & FOPEN)
Note that os.fstat() fails on Windows with OSError(EBADF, ...).

This can probably be fixed by using _PyVerify_fd().

Many os functions started to accept file descriptors. I don't know how many are available on Windows, but IMO _PyVerify_fd() could be used for all of them; it's a no-op macro on Unix anyway.

New changeset 62b9bfbc3356 by Richard Oudkerk in branch 'default':
Issue #15261: Stop os.stat(fd) crashing on Windows when fd not open.
http://hg.python.org/cpython/rev/62b9bfbc3356 

> Many os functions started to accept file descriptors. 
> I don't know how many are available on Windows...
On Windows os.stat() seems to be the only one:
 >>> os.supports_fd
 {<built-in function stat>}

The 62b9bfbc3356 changeset does more than add the guard against invalid file handles; it also adds documentation to os.path.exists documenting that it now accepts path-as-int-fd. While this modification is fine in principle I would have preferred it was submitted separately.

Sorry about that...