Does Python's open() read an unflushed buffer?

My knowledge of how file buffers work is fairly weak, so forgive the simple mindedness of this question. I have a piece of code in Python that waits for a file to appear, then reads it.

while 1:
    try:
        f = open('file.txt')
        f.close()
        print "Message received."
        break
    except:
        time.sleep(0.3)

Where file.txt is a file written by another program. However, I've a suspicion that Python is reading the file and then closing the handle before the file file.txt is completely written (that is, before I even call close() in the program that is writing the file). Should this even be possible?

If it is the case, is there a way 开发者_开发技巧I can detect (in the reading program listed above) whether or not the buffers have been flushed before I call f.close()?

Don't use a while-loop for this purpose. There is a better way, though the details depend on your OS.

Linux has inotify. You can write a simple pyinotify script to watch the directory and react to IN_CLOSE_WRITE events (which occur only after the file has been closed).

OSX (FSEvents) and Windows have similar facilities.

Python will not stop you from reading a file that another process has not completed writing.

If I'm waiting for a file from another process, I usually have that process write to a different file name. Once that process has finished writing it closes and renames the file to the file name i'm expecting. That way I don't get a partial file.

To check whether a file exists, use os.path.exists or maybe os.path.isfile if you require it to be a "regular" file.

On Unix, a directory entry will be created when the file is opened for write/append. But bytes written to a file will not actually show up there (from the point of view of a reader) until I/O is flushed. There is no normal way of asking whether there is "pending" I/O on a file; at least not on Unix.

In general Linux has no problem whatsoever with one process opening a file for read while another process has it open for writing. Think about doing things like tail -f $SOME_FILE to follow updates to a file in real time; that could not possibly work if attempting to open the file for reading had to block and wait until the writer closed the file.