"what exactly the python's file.flush() is doing?" says you should first f.flush() and then os.fsync(f.fileno()) to make sure the data are written to the disk.
Furthermore, "does close() imply flush() in Python?" claims that f.close() implies a flush().
Now, the question is: should I do
os.fsync(f.fileno())
f.close()
or, does f.close() also implies an os.fsync()?
Here are the doc of Python IO, the doc of Python file close, and the source code. A quick search of 'fsync' returned no relevant info.
I checked myself by stracing a test script and no it does not imply an fsync. f.close() implies a f.flush() because it has to. f.flush() writes the internal (user space) buffers to the associated file descriptor and you can not write to a file descriptor after closing it. However writing to the kernel buffers does not guarantee durability and this is where fsync comes into play.