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I have 2 scripts script1.py and script2.py in the same folder ,script1.py calls script2.py using Popen(See code below for details),issue is that the prints coming from script2.py is not being captured in script1.py,print output and print error doesn't print a thing in the code below? what am I missing here? how do I capture the prints from script2.py?

script1.py

import subprocess
from subprocess import Popen, PIPE, STDOUT
def func1 ():
    cmd = "python script2.py"
    proc = Popen(cmd.split(' '), stdout=PIPE, stderr=PIPE)
    (output, error) = proc.communicate()
    print output
    print error

func1()
print "Done.."

script2.py

import sys
print "ERROR:port not detected"
sys.exit(29)

OUTPUT:-

C:\Dropbox>python script1.py
ERROR:port not detected


Done..
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7
  • is there more to these scripts? "script1.py" never calls func1() and "script2.py" never defines a function func1() but does call it.. Commented Oct 12, 2016 at 16:37
  • Thanks for pointing...missed to add a piece...updated both script1.py and script2.py Commented Oct 12, 2016 at 16:39
  • there is no error here then... Commented Oct 12, 2016 at 16:41
  • issue is that the prints coming from script2.py is not being captured in script1.py,print output and print error doesn't print a thing in the code Commented Oct 12, 2016 at 16:43
  • I ran it through debug on my computer and it runs as expected... Commented Oct 12, 2016 at 16:46

2 Answers 2

Reset to default
3

Edited answer based on comments

Looks like after the edits you made to the original question, Your code is working correctly. I just put output= in front of print statement to check that.

import subprocess
from subprocess import Popen, PIPE, STDOUT
def func1 ():
    cmd = "python script2.py"
    proc = Popen(cmd.split(' '), stdout=PIPE, stderr=PIPE)
    (output, error) = proc.communicate()
    print "output=",output
    print error

func1()
print "Done.."

** OUTPUT: **

Python 2.7.9 (default, Dec 10 2014, 12:24:55) [MSC v.1500 32 bit (Intel)] on win32
Type "copyright", "credits" or "license()" for more information.
>>> ================================ RESTART ================================
>>> 
output= ERROR:port not detected



Done..
>>>
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6 Comments

Folks, if you are down voting, please explain why otherwise it's futile.
@user2125827: If this solutions solves the problem, pls consider accepting (and up-voting) to keep away unwanted down-voters.
Downvoted because it makes it seem like the fix was to replace communicate (in the question) with stdout.read. This does not change anything. communicate was already doing the same job correctly. The aspect of your answer that "fixed" the OP's problem was that you flagged the output with 'output=' and made him realize that in fact there was no problem to start with. The original OP code would also be "fixed" (i.e. prevent the OP from mistakenly believing it's not working) when you do this.
using output = proc.stdout.read() may (if you have a huge output) overflow the buffer.. additionally if stdout=PIPE is defined, that buffer can overflow and block if it is not read as well as stdin. see this answer as well as the comment discussion. given this if it is acceptable to read the output only once the process has terminated, it's generally preferable to use communicate for simplicity's sake.
My answer was based on OP's original question which he has subsequently edited. After edits, yes I agree his code is working correctly. Hence, I've modified my answer accordingly. Although, I must mention that changing questions adds to the confusion , especially all the comments are based on previous question and previous solution.
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1

your script is in fact working as intended. You likely are expecting a traceback to be printed to stderr of your subprocess, but that's not how sys.exit() works.

script1.py

import subprocess
from subprocess import Popen, PIPE, STDOUT
def func1 ():
    cmd = "python script2.py"
    proc = Popen(cmd.split(' '), stdout=PIPE, stderr=PIPE)
    (output, error) = proc.communicate()
    print output[::-1] #prints reversed message proving print is called from script1 not script2
    print error #prints nothing because there is no error text (no error was raised only system exit)
    print 'return status: '+str(proc.returncode) #this is what sys.exit() modifies
func1()
print "Done.. YAY" #done prints after call

script2.py

import sys
print "ERROR:port not detected"
sys.exit(29) #does not print traceback (or anything) only sets return code
print "Done.." #never prints because python interpreter is terminated

when script1 calls script2 as a subprocess, script2 prints it's first statement to the stdout pipe, then exits with a return code of 29. A system exit with a return code of 0 is viewed as a successful return, so unless you specifically call something that raises an error, nothing will print to the stderr pipe. the return code can however be determined from the attribute returncode of your proc.

running >python script1.py yields:

detceted ton trop:RORRE

return status: 29
Done.. YAY
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2 Comments

for fun, add print 1/0 just before sys.exit(29) to see an example of what would happen if an exception was raised..
or try omitting the sys.exit statement entirely and see what that changes

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