How to get output from subprocess.Popen(). proc.stdout.readline() blocks, no data prints out
Solution 1
You obviously can use subprocess.communicate but I think you are looking for real time input and output.
readline was blocked because the process is probably waiting on your input. You can read character by character to overcome this like the following:
import subprocess
import sys
process = subprocess.Popen(
cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE
)
while True:
out = process.stdout.read(1)
if out == '' and process.poll() != None:
break
if out != '':
sys.stdout.write(out)
sys.stdout.flush()
Solution 2
Nadia's snippet does work but calling read with a 1 byte buffer is highly unrecommended. The better way to do this would be to set the stdout file descriptor to nonblocking using fcntl
fcntl.fcntl(
proc.stdout.fileno(),
fcntl.F_SETFL,
fcntl.fcntl(proc.stdout.fileno(), fcntl.F_GETFL) | os.O_NONBLOCK,
)
and then using select to test if the data is ready
while proc.poll() == None:
readx = select.select([proc.stdout.fileno()], [], [])[0]
if readx:
chunk = proc.stdout.read()
print chunk
She was correct in that your problem must be different from what you posted as Caller.py and Test_Pipe.py do work as provided.
Solution 3
Test_Pipe.py
buffers its stdout by default so proc
in Caller.py
doesn't see any output until the child's buffer is full (if the buffer size is 8KB then it takes around a minute to fill Test_Pipe.py
's stdout buffer).
To make the output unbuffered (line-buffered for text streams) you could pass -u
flag to the child Python script. It allows to read subprocess' output line by line in "real-time":
import sys
from subprocess import Popen, PIPE
proc = Popen([sys.executable, "-u", "Test_Pipe.py"], stdout=PIPE, bufsize=1)
for line in iter(proc.stdout.readline, b''):
print line,
proc.communicate()
See links in Python: read streaming input from subprocess.communicate() on how to solve the block-buffering issue for non-Python child processes.
Solution 4
To avoid the many problems that can always arise with buffering for tasks such as "getting the subprocess's output to the main process in real time", I always recommend using pexpect for all non-Windows platform, wexpect on Windows, instead of subprocess
, when such tasks are desired.
wearetherock
Updated on July 17, 2022Comments
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wearetherock almost 2 years
I want output from execute Test_Pipe.py, I tried following code on Linux but it did not work.
Test_Pipe.py
import time while True : print "Someting ..." time.sleep(.1)
Caller.py
import subprocess as subp import time proc = subp.Popen(["python", "Test_Pipe.py"], stdout=subp.PIPE, stdin=subp.PIPE) while True : data = proc.stdout.readline() #block / wait print data time.sleep(.1)
The line
proc.stdout.readline()
was blocked, so no data prints out. -
jfs almost 11 yearsIt won't produce the output any sooner than the code in the question due to the block-buffering issue. See my answer.
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jfs almost 11 years
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Derrick Petzold almost 9 yearsAgain you are using 1 byte buffer which is incredibly inefficient.
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Derrick Petzold almost 9 yearsThe code does produce output in realtime. Did you have a problem with it?
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jfs almost 9 yearsfollow the link in the comment.
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jfs almost 9 years@DerrickPetzold: wrong.
bufsize=1
means "line-buffered". It uses the same buffer size asbufsize=-1
. You could have found out that both of your comments are wrong if you were actually to run the code (compare time performance and measure the time before the first byte is read) -
sherrellbc over 7 yearsWhat does it mean for data "to be ready?" Is it newline delineated or just if at least one byte of data is ready (even if the producer of such data is still writing to its stdout)?
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sherrellbc over 7 years@me, J.F.Sebastian's answer below addresses this.
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pelos over 7 yearsi like this way of doing it because you can print directly to screen and also store the results of the popen to do something with, and later on check error code etc... thumbs up =) i run it with out the bufsize and does work fine for me in my code.
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jfs about 6 yearsto be clear: neither yours nor Nadia's answers produce the output in "real-time" due to the internal buffers in the child process.
os.O_NONBLOCK
in your code (in the parent process) has no effect on internal stdio buffers in the child process. For example, running your code produce 4K chunks instead oflen(chunk) == 13
(one line). To see the difference, pass-u
option (run[sys.executable or 'python2', '-u', 'Test_Pipe.py']
command).