When an open attempts to access a directory as data, the error code from WinAPI CreateFileW is ERROR_ACCESS_DENIED (5), which maps to C EACCES and thus raises Python PermissionError. 

Python's io.FileIO() type could handle an EACCES error in Windows by checking for a directory. There's a race condition to consider in which an inaccessible file is replaced by a directory, but that seems to be a trivial concern.

io.FileIO() could also use the undocumented _O_OBTAIN_DIR (0x2000) open flag. This opens with backup semantics, which allows opening directories. Then, as already implemented in POSIX, fail with EISDIR if a directory is opened. For example:

    >>> fd = os.open('C:/Temp', 0x2000)
    >>> stat.S_ISDIR(os.fstat(fd).st_mode)
    True