Is there any way to get the effect of running python -u from within my code? Failing that, can my program check if it is running in -u mode and exit with an error message if not? This is on Linux (Ubuntu 8.10 Server).
4 Answers
The best I could come up with:
>>> import os
>>> import sys
>>> unbuffered = os.fdopen(sys.stdout.fileno(), 'w', 0)
>>> unbuffered.write('test')
test>>>
>>> sys.stdout = unbuffered
>>> print 'test'
test
Tested on GNU/Linux. It seems it should work on Windows too. If I knew how to reopen sys.stdout, it would be much easier:
sys.stdout = open('???', 'w', 0)
References:
http://docs.python.org/library/stdtypes.html#file-objects
http://docs.python.org/library/functions.html#open
http://docs.python.org/library/os.html#file-object-creation
[Edit]
Note that it would be probably better to close sys.stdout before overwriting it.
3 Comments
Thomas Ahle
stdout still lives in sys.__stdout__
vbo
Note that this doesn't work in Py3K.
ValueError: can't have unbuffered text I/O
xjcl
The answers on this question are terribly out of date. Please refer to the linked question for Python 3.3+ answers: stackoverflow.com/q/107705/2111778
You could always pass the -u parameter in the shebang line:
#!/usr/bin/python -u
7 Comments
Tobu
See title: 1. OP knows about it; 2. wants to do it programmatically.
mikewaters
The author implies command-line usage, not indicating familiarity with the alternate invocation offered by the shell (shebang). Thanks for your helpful feedback though.
Aaron McDaid
This doesn't seem to work with the "env trick".
#!/usr/bin/env python -u. I get the following error /usr/bin/env: python -u: No such file or directory. If I remove the -u it works again.Aaron McDaid
(me again!). Here is a solution to that problem I discussed in my two comments above: stackoverflow.com/questions/17458528/why-does-this-snippet-work . (But yes, this isn't related to the original question here)
Nico Villanueva
As Aaron says, plainly adding the option to the shebang doesn't work. Instead, pass the
-S option to the env command, like so: #!/usr/bin/env -S python3 -u |
Assuming you're on Windows:
msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
... and on Unix:
fl = fcntl.fcntl(sys.stdout.fileno(), fcntl.F_GETFL)
fl |= os.O_SYNC
fcntl.fcntl(sys.stdout.fileno(), fcntl.F_SETFL, fl)
(Unix copied in from commented solution, rather than linking.)
4 Comments
Martin DeMello
sorry, forgot to add that. on linux (ubuntu)
S.Lott
@Martin DeMello: Please do not add new facts in comments. Please update your question with new facts. New facts in comments are hard to find.
Tobu
Supported on unix, linux, and anything posix: stackoverflow.com/questions/107705/python-output-buffering/…
Romuald Brunet
Note that this does not work with python2 (no error, output is still buffered). Found out when running a script with the wrong interpreter
EDIT (Oct 2020). As pointed out in a note to this answer, in Python3, stderr is buffered too.
You might use the fact that stderr is never buffered and try to redirect stdout to stderr:
import sys
#buffered output is here
doStuff()
oldStdout = sys.stdout
sys.stdout = sys.stderr
#unbuffered output from here on
doMoreStuff()
sys.stdout = oldStdout
#the output is buffered again
doEvenMoreStuff()
3 Comments
Martin DeMello
a hack, but a very cute one :) doesn't really work if you need your stderr separate but nice idea
Boris Gorelik
thanks. as to the separation: you can't get it all, can you :)
Don Hatch
"stderr is never buffered" - no longer true; in python3, stderr is line buffered if to a tty, fully buffered if to a file or pipe.
PYTHONUNBUFFEREDenv var to any non-empty string. This is NOT in your code — must be set before python interpreter starts — but is easier than modifying whatever launches your script to add-u.