None in redirected stdout

Question

I have this part of following code:

p = subprocess.Popen(['C:/Python27/python.exe', '-m', 'robot', '-d', logs_directory, input_file], stdout=subprocess.PIPE)
for line in iter(p.stdout.readline, ''):
    output = sys.stdout.write(line)
    #sys.stdout.write(line)
print "\n\n"
print "************************************************"
print  output
print "\n\n"
print "************************************************"
print "\n\n"

But 'output' variable in console shows 'none'

************************************************
None



************************************************

What i am doing wrong here?

related: [Python subprocess get children's output to file and terminal?](http://stackoverflow.com/q/4984428/4279) — jfs, Apr 22 '16 at 19:21

score 3 · Accepted Answer · edited Apr 22 '16 at 10:36

3

sys.stdout.write(line) returns the number of characters written on stdout.

You could write your loop like this:

output = ""
for line in iter(p.stdout.readline, ''):
    output += line
    sys.stdout.write(line)

imho you can also remove the sys.stdout.write(line) part. But I don't know what you are meant to do with it so...

edited Apr 22 '16 at 10:36

bashrc

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answered Apr 22 '16 at 08:00

salomonderossi

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sys.stdout.write(line) does return a value. It returns the number of characters written on stdout. – bashrc Apr 22 '16 at 10:10
1- `sys.stdout.write()` returns `None` on Python 2 by default. 2- `output += line` may produce quadratic behavior on some Python implementations. You could use `lines = []; ... lines.append(line) ... output = ''.join(lines)` instead (linear behavior). – jfs Apr 22 '16 at 19:26

score 1 · Answer 2 · answered Apr 22 '16 at 08:01

1

Try output += line instead of output = sys.stdout.write(line)

answered Apr 22 '16 at 08:01

pelya

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dokelung · Answer 3 · 2016-04-22T08:28:04.273

0

You can also try function check_output.

Here is an example:

>>> subprocess.check_output(["echo", "Hello World!"])
'Hello World!\n'

You can try:

cmd = ['C:/Python27/python.exe', '-m', 'robot', '-d', logs_directory, input_file]
output = subprocess.check_output(cmd)
print output

In most of the cases, it can do the job simply.

For details, you may want to reference to subprocess.check_output

edited Apr 22 '16 at 08:28

answered Apr 22 '16 at 08:16

dokelung

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Correct, but i also want to print results to console, check_output is not sufficient in this case. – user2678074 Apr 22 '16 at 08:26
I don't understand why you think `check_output` is not sufficient. You just assign its return value to a variable then print to console. – dokelung Apr 22 '16 at 08:35
that won't be 'realtime' print. I'll get console output when subprocess will end. I want to have console output so i can see progress sof subprocess – user2678074 Apr 22 '16 at 08:40
OK, I see. Thanks! – dokelung Apr 22 '16 at 08:53

None in redirected stdout

3 Answers3