Write a simple python script test.py:
import time
print "begin"
time.sleep(10)
print "stop"
In bash, run
python test.py > log.txt
What I observe is that both "begin" and "stop" appear in log.txt at the same time, after 10 seconds.
Is this expected behavior?
Have you tried calling python with the -u option, which "forces stdin, stdout and stderr to be totally unbuffered" =>
python -u test.py > log.txt
You need to flush the buffer after printing.
import sys
import time
print "begin"
sys.stdout.flush()
time.sleep(10)
print "end"
sys.stdout.flush()
Or in Python 3:
# This can also be done in Python 2.6+ with
# from __future__ import print_function
import time
print("begin", flush=True)
time.sleep(10)
print("end", flush=True)
This is because the actual writing happens only when a buffer is full, or when the file is closed. When the program ends (after 10s), the buffer is emptied, the data gets written and the file closed.
Related
I have the following code in file abc.py:
import subprocess
def evaluate():
for i in range(5):
print("Printing", i)
subprocess.call(['ls', '-lrt'])
print("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~")
evaluate()
Now when I call using python abc.py > out.dat, the output file contains the result of 'ls -lrt' five times which is followed by printing statements in the code.
Why is it happening so and what should I do if I need to get output as:
printing 0
(results of ls -lrt here)
~~~~~~~~~~~~~~~~~~~~~~~
printing 1
.
.
.
and so on..?
Thank you..
You need to flush your stream before you call a subprocess:
import subprocess, sys
def evaluate():
for i in range(5):
print("Printing", i)
sys.stdout.flush()
subprocess.call(['ls', '-lrt'])
print("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~")
evaluate()
Flushing takes automatically place line-by-line as long as you write to a terminal (without redirection). Then you do not notice the problem because the print() flushes at the newline.
As soon as you redirect your output to a file the print() statement notices this and flushes automatically only when a certain buffer is full (and at termination time).
The subprocess (ls) does the same but uses its own buffer. And it terminates first, so its output is in the file first.
Usually I'd like to run my python code in linux as the following:
nohup python test.py > nohup.txt 2>&1 &
in the file test.py, I often use print to print out some messages to stdout. But actually I have to wait very long time then could see the messages were printed out to nohup.txt. How can I make it print out quickly.
You could call flush on stdout. If it is possible and practical for you to adjust your code to flush your buffers after the print call, in test.py:
from sys import stdout
from time import sleep
def log():
while True:
print('Test log message')
# flush your buffer
stdout.flush()
sleep(1)
While running this logging test, you can check the nohup.txt file and see the messages being printed out in realtime.
just make sure you have coreutils installed
stdbuf -oL nohup python test.py > nohup.txt 2>&1 &
this just sets buffering to off for this command ... you should see immediate output
(you might need nohup before stdbuf ... im not sure exactly)
alternatively ... just put at the top of test.py
import sys
sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)
I have two scripts which are connected by Unix pipe. The first script writes strings to standard out, and these are consumed by the second script.
Consider the following
# producer.py
import sys
import time
for x in range(10):
sys.stdout.write("thing number %d\n"%x)
sys.stdout.flush()
time.sleep(1)
and
# consumer.py
import sys
for line in sys.stdin:
print line
Now, when I run: python producer.py | python consumer.py, I expect to see a new line of output each second. Instead, I wait 10 seconds, and I suddenly see all of the output at once.
Why can't I iterate over stdin one-item-at-a-time? Why do I have to wait until the producer gives me an EOF before the loop-body starts executing?
Note that I can get to the correct behavior if I change consumer.py to:
# consumer.py
import sys
def stream_stdin():
line = sys.stdin.readline()
while line:
yield line
line = sys.stdin.readline()
for line in stream_stdin():
print line
I'm wondering why I have to explicitly build a generator to stream the items of stdin. Why doesn't this implicitly happen?
According to the python -h help message:
-u Force stdin, stdout and stderr to be totally unbuffered. On systems where it matters, also put stdin, stdout and stderr in
binary mode. Note that there is internal buffering in xread‐
lines(), readlines() and file-object iterators ("for line in
sys.stdin") which is not influenced by this option. To work
around this, you will want to use "sys.stdin.readline()" inside
a "while 1:" loop.
I got a simple python script which should read from stdin.
So if I redirect a stdout of a program to the stdin to my python script.
But the stuff that's logged by my program to the python script will only "reach" the python script when the program which is logging the stuff gets killed.
But actually I want to handle each line which is logged by my program as soon as it is available and not when my program which should actually run 24/7 quits.
So how can I make this happen? How can I make the stdin not wait for CTRL+D or EOF until they handle data?
Example
# accept_stdin.py
import sys
import datetime
for line in sys.stdin:
print datetime.datetime.now().second, line
# print_data.py
import time
print "1 foo"
time.sleep(3)
print "2 bar"
# bash
python print_data.py | python accept_stdin.py
Like all file objects, the sys.stdin iterator reads input in chunks; even if a line of input is ready, the iterator will try to read up to the chunk size or EOF before outputting anything. You can work around this by using the readline method, which doesn't have this behavior:
while True:
line = sys.stdin.readline()
if not line:
# End of input
break
do_whatever_with(line)
You can combine this with the 2-argument form of iter to use a for loop:
for line in iter(sys.stdin.readline, ''):
do_whatever_with(line)
I recommend leaving a comment in your code explaining why you're not using the regular iterator.
It is also an issue with your producer program, i.e. the one you pipe stdout to your python script.
Indeed, as this program only prints and never flushes, the data it prints is kept in the internal program buffers for stdout and not flushed to the system.
Add sys.stdout.flush() call right after you print statement in print_data.py.
You see the data when you quit the program as it automatically flushes on exit.
See this question for explanation,
As said by #user2357112 you need to use:
for line in iter(sys.stdin.readline, ''):
After that you need to start python with the -u flag to flush stdin and stdout immediately.
python -u print_data.py | python -u accept_stdin.py
You can also specify the flag in the shebang.
I have a python script test.py:
print "first"
import os
os.system("echo second")
On linux command line I execute
python test.py
which returns:
first
second
I then execute
python test.py > test.out; cat test.out
which returns
second
first
What about redirecting the output makes the os.system call print before the print statement??
When you're outputting to a pipe, Python buffers your output that's written to sys.stdout and outputs it after a flush, or after it's overflown, or upon closing (when the program exits). While it'll buffer the print calls, system calls output directly into stdout and their output won't be buffered. That's why you're seeing such precedence. To avoid that, use python -u:
python -u test.py > test.out; cat test.out
See more info here.
EDIT: explanation on when the buffer will be flushed.
Another way to prevent the os buffering is to flush the output after the first print:
#!/usr/bin/env python
import sys
print "first"
sys.stdout.flush()
import os
os.system("echo second")
When the output of the python script is a tty, its output is line buffered. When the output is a regular file, the output is block buffered.