Bug 3.11.x behavioral, open file buffers not flushed til file closed.

[Weatherby,Gerard]

Add f.reconfigure it you want line buffering in your example:

f = open("abc", "w")
f.reconfigure(line_buffering=True)
for i in range(50000):
   f.write(str(i) + "\n")

More Pythonic would be:

with open("abc", "w") as f:
   for i in range(5000):
      print(i,file=f)

From: Python-list <python-list-bounces+gweatherby=uchc.edu at python.org> on behalf of aapost <aapost at idontexist.club>
Date: Sunday, March 5, 2023 at 6:33 PM
To: python-list at python.org <python-list at python.org>
Subject: Bug 3.11.x behavioral, open file buffers not flushed til file closed.
*** Attention: This is an external email. Use caution responding, opening attachments or clicking on links. ***

I have run in to this a few times and finally reproduced it. Whether it
is as expected I am not sure since it is slightly on the user, but I can
think of scenarios where this would be undesirable behavior.. This
occurs on 3.11.1 and 3.11.2 using debian 12 testing, in case the
reasoning lingers somewhere else.

If a file is still open, even if all the operations on the file have
ceased for a time, the tail of the written operation data does not get
flushed to the file until close is issued and the file closes cleanly.

2 methods to recreate - 1st run from interpreter directly:

f = open("abc", "w")
for i in range(50000):
   f.write(str(i) + "\n")

you can cat the file and see it stops at 49626 until you issue an f.close()

a script to recreate:

f = open("abc", "w")
for i in range(50000):
   f.write(str(i) + "\n")
while(1):
   pass

cat out the file and same thing, stops at 49626. a ctrl-c exit closes
the files cleanly, but if the file exits uncleanly, i.e. a kill command
or something else catastrophic. the remaining buffer is lost.

Of course one SHOULD manage the closing of their files and this is
partially on the user, but if by design something is hanging on to a
file while it is waiting for something, then a crash occurs, they lose a
portion of what was assumed already complete...
--
https://urldefense.com/v3/__https://mail.python.org/mailman/listinfo/python-list__;!!Cn_UX_p3!kBYMol9JmMVwZD0iSoeeR1fYTiX8DEG-V4LBm4aAw4IJQ6Am4Ql_HYRZOeO8XK3kZvq2_adnid-FeoHr37Tw2I7k$<https://urldefense.com/v3/__https:/mail.python.org/mailman/listinfo/python-list__;!!Cn_UX_p3!kBYMol9JmMVwZD0iSoeeR1fYTiX8DEG-V4LBm4aAw4IJQ6Am4Ql_HYRZOeO8XK3kZvq2_adnid-FeoHr37Tw2I7k$>