-------- Forwarded Message --------
Subject: Re: Unbuffered stderr in Python 3
Date: Tue, 03 Nov 2015 18:03:51 +0000
From: George Trojan <george.tro...@noaa.gov>
To: python-list@python.org
On 11/03/2015 05:00 PM, python-list-requ...@python.org wrote:
On Mon, 02 Nov 2015 18:52:55 +1100, Steven D'Aprano wrote:
In Python 2, stderr is unbuffered.
In most other environments (the shell, C...) stderr is unbuffered.
It is usually considered a bad, bad thing for stderr to be buffered. What
happens if your application is killed before the buffer fills up? The
errors in the buffer will be lost.
So how come Python 3 has line buffered stderr? And more importantly, how
can I turn buffering off?
It's probably related to the fact that std{in,out,err} are Unicode
streams.
> type(sys.stderr)
<class '_io.TextIOWrapper'>
> type(sys.stderr.buffer)
<class '_io.BufferedWriter'>
> type(sys.stderr.buffer.raw)
<class '_io.FileIO'>
It appears that you can turn it off with:
sys.stderr = io.TextIOWrapper(sys.stderr.buffer.raw)
or:
sys.stderr = io.TextIOWrapper(sys.stderr.detach().detach())
This results in a sys.stderr which appears to work and whose
.line_buffering property is False.
This does set line buffering, but does not change the behaviour:
(devenv-3.4.1) dilbert@gtrojan> cat x.py
import sys
import time
if sys.version>'3':
import io
sys.stderr = io.TextIOWrapper(sys.stderr.detach().detach())
#sys.stderr = io.TextIOWrapper(sys.stderr.buffer.raw)
print(sys.stderr.line_buffering)
sys.stderr.write('a')
time.sleep(10)
This is python2.7.5. a is printed before ^C.
(devenv-3.4.1) dilbert@gtrojan> /bin/python x.py
a^CTraceback (most recent call last):
Here buffer is flushed on close, after typing ^C.//
(devenv-3.4.1) dilbert@gtrojan> python x.py
False
^CaTraceback (most recent call last):
George
Found it. write_through must be set to True.
(devenv-3.4.1) dilbert@gtrojan> cat x.py
import sys
import time
if sys.version>'3':
import io
sys.stderr = io.TextIOWrapper(sys.stderr.detach().detach(),
write_through=True)
#sys.stderr = io.TextIOWrapper(sys.stderr.buffer.raw)
print(sys.stderr.line_buffering)
sys.stderr.write('a')
time.sleep(10)
(devenv-3.4.1) dilbert@gtrojan> python x.py
False
a^CTraceback (most recent call last):
/
/
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