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I'm not sure if you are referring to Python's C-level fdopen or GNU libc fdopen.
GNU libc fdopen does not consume file descriptor on error:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
int main(int argc, char** argv)
{
 int fd = -1;
 int rv = 0;
 FILE* fh = NULL;
 if (argc<3) return 1;
 errno = 0;
 fd = open(argv[1], O_RDONLY);
 printf("got fd %d errno %d text %s\n", fd, errno, strerror(errno));
 errno = 0;
 fh = fdopen(fd, argv[2]);
 printf("got fh %x errno %d text %s\n", fh, errno, strerror(errno));
 errno = 0;
 rv = close(fd);
 printf("got rv %d errno %d text %s\n", rv, errno, strerror(errno));
}
[dima@bmg ~]$ ./a.out /etc/passwd w
got fd 4 errno 0 text Success
got fh 0 errno 22 text Invalid argument
got rv 0 errno 0 text Success
To be fair, GNU libc fdopen doesn't consider it an error to use a file descriptor that refers to a directory, which is the crux of this bug.
Anyhow, point is the semantics change your patch brings in sets Python 2.7+ in contrast with both Python 3.x and GNU libc. 
Perhaps if it's too hard to implement properly, it's better to leave this issue as won't fix / technical limitation?