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.
Related
I wanted to run a test function called test.counttest() that counts up to 10.
def counttest():
x = 0
for x in range(0,3):
x = x+1
print("Number: "+ str(x))
time.sleep(1)
I want to call just the function from the command line OR from subprocess popen. Not write the function, just call it. Everything I have google keeps bringing me back to how I can write a function from the command line which is NOT what I need.
I need to specifically run a function from subprocess popen so I can get the stdout in a forloop that can then be sent to a flask socket. (This is required)
Main point - How can Call (not write) a function from the command line or from subprocess?
Not this:
python -c 'import whatever then add code'
But something like this:
python "test.counttest()"
or like this:
subprocess.Popen(['python', ".\test.counttest()"],stdout=subprocess.PIPE, bufsize=1,universal_newlines=True)
EDIT:
This is for #Andrew Holmgren. Consider the following script:
def echo(ws):
data = ws.receive()
with subprocess.Popen(['powershell', ".\pingtest.ps1"],stdout=subprocess.PIPE, bufsize=1,universal_newlines=True) as process:
for line in process.stdout:
line = line.rstrip()
print(line)
try:
ws.send(line+ "\n")
except:
pass
this works perfectly for what I need as it:
takes the script's stdout and send's it to the ws.send() function which is a websocket.
However I need this same concept for a function instead. The only way I know how to get the stdout easily is from using subprocess.popen but if there is another way let me know. This is why I am trying to make a hackjob way of running a function through the subprocess module.
The question of Run python function from command line or subprocess popen relates in the fact that if I can get a function to run from subprocess, then I know how to get the stdout for a websocket.
Actually you have really a lot of questions inside this one.
How can I send output of function line-by-line to another one (and/or websocket)? Just avoid writing to stdout and communicate directly. yield (or other generator creation methods) are intended exatly for that.
import time
def counttest():
for i in range(10):
yield f'Item {i}'
time.sleep(1)
def echo(ws):
# data = ws.receive()
for row in counttest():
ws.send(row)
How to call a function func_name defined in file (suppose it's test.py) from command line? Being in directory with test.py, do
$ python -c 'from test import func_name; func_name()'
How to read from sys.stdout? The easiest will be to replace it with io.StringIO and restore thing back later.
from contextlib import redirect_stdout
import io
def echo(ws):
f = io.StringIO()
with redirect_stdout(f):
counttest()
output = f.getvalue()
ws.send(output)
It will return only after call_function(), so you cannot monitor real-time printed items.
Regarding
I need the stdout
I can say, that I'm sure your question is X-Y problem, thus I try to suggest alternatives. Solution you want will also work, but it's awkward. This will exactly run function counttest defined in test.py, capture its output and send it line-by-line to websocket. It will process output immediately when new line arrives. Note -u flag on python call (unbuffered), it's important.
import subprocess
import shlex
def echo(ws):
data = ws.receive()
with subprocess.Popen(shlex.split("python -u -c 'from test import counttest; counttest()'"),
stdout=subprocess.PIPE,
bufsize=1,
universal_newlines=True) as process:
for line in iter(process.stdout.readline, ''):
line = line.rstrip()
if not line:
break
print(line)
try:
ws.send(line + "\n")
except:
pass
In your Document folder create a folder temp:
/My Documents/temp
Save these few lines as worker.py Python scripts:
import time
from datetime import datetime
for i in range(10):
print '%s...working on iteration %s' % (datetime.now(), i)
time.sleep(0.2)
print '\nCompleted!\n'
Save the code below as caller.py:
import subprocess
cmd = ['python', 'worker.py']
stdout = subprocess.check_output(cmd)
print stdout
(Please note that both Python scripts were saved in to the same folder).
Now using the OS X Terminal or Windows CMD window change the current directory to the folder you created:
cd /My Documents/temp
Now run:
python caller.py
The process takes 2 seconds to complete. When completed it prints out the entire progress log all at once:
2018-01-20 07:52:14.399679...working on iteration 0
...
2018-01-20 07:52:16.216237...working on iteration 9
Completed!
Instead of getting the log printed (all at once after the process has been already completed), I would like the have a real-time progress update. I would like to get every printed line from the process at the same moment it occured.
So, when I run python worker.pycommand it will give me line by line update happening in a real time. How to achieve it?
To get a real-time feed from the subprocess you can use this code in the caller.py
import time
import subprocess
# Start worker process
p = subprocess.Popen(['python', '-u', 'worker.py'], stdout=subprocess.PIPE)
# Loop forever
while True:
# Get new line value
l = p.stdout.readline()
# Stop looping if the child process has terminated
if p.poll() is not None:
break
# Print the line
print l
Note the -u in the subprocess.Popen, you need unbuffered stdout.
https://docs.python.org/3/using/cmdline.html#cmdoption-u
With readline() you are reading a single line per time from the subprocess output. Be aware when the subprocess prints '\nCompleted!\n' you will read it in three loops.
https://docs.python.org/3/tutorial/inputoutput.html#methods-of-file-objects
In the example, the loop will run until the subprocess will terminate.
https://docs.python.org/3/library/subprocess.html#subprocess.Popen.poll
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.