I propose a fix: static FILE * open_exclusive(char *filename, mode_t mode) { #if defined(O_EXCL)&&defined(O_CREAT)&&defined(O_WRONLY)&&defined(O_TRUNC) /* Use O_EXCL to avoid a race condition when another process tries to write the same file. When that happens, our open() call fails, which is just fine (since it's only a cache). XXX If the file exists and is writable but the directory is not writable, the file will never be written. Oh well. */ int fd; (void) unlink(filename); fd = open(filename, O_EXCL|O_CREAT|O_WRONLY|O_TRUNC #ifdef O_BINARY |O_BINARY /* necessary for Windows */ #endif #ifdef __VMS , mode, "ctxt=bin", "shr=nil" #elif defined(MS_WINDOWS)	, mode | _S_IWRITE #else , mode #endif ); if (fd < 0 ) return NULL; return fdopen(fd, "wb"); #else /* Best we can do -- on Windows this can't happen anyway */ return fopen(filename, "wb"); #endif } ---------------------- so doesn't matter what the .py file permission is under windows, the .pyc file will always have both read and write permissions.