does close() imply flush() in Python?

Question

In Python, and in general - does a close() operation on a file object imply a flush() operation?

Answer 1

Yes. It uses the underlying close() function which does that for you (source).

Answer 2

NB: close() and flush() won't ensure that the data is actually secure on the disk. It just ensures that the OS has the data == that it isn't buffered inside the process.

You can try sync or fsync to get the data written to the disk.

Answer 3

Yes, in Python 3 this is finally in the official documentation, but is was already the case in Python 2 (see Martin's answer).

Answer 4

filehandle.close does not necessarily flush. Surprisingly, filehandle.flush doesn't help either---it still can get stuck in the OS buffers when Python is running. Observe this session where I wrote to a file, closed it and Ctrl-Z to the shell command prompt and examined the file:

$  cat xyz
ghi
$ fg
python

>>> x=open("xyz","a")
>>> x.write("morestuff\n")
>>> x.write("morestuff\n")
>>> x.write("morestuff\n")
>>> x.flush
<built-in method flush of file object at 0x7f58e0044660>
>>> x.close
<built-in method close of file object at 0x7f58e0044660>
>>> 
[1]+  Stopped                 python
$ cat xyz
ghi

Subsequently I can reopen the file, and that necessarily syncs the file (because, in this case, I open it in the append mode). As the others have said, the sync syscall (available from the os package) should flush all buffers to disk but it has possible system-wide performance implications (it syncs all files on the system).