Getting the output of a python subprocess

Question

I'm trying to capture the output here. If, at the python prompt, I run

p = subprocess.Popen(["/path/to/search_by_hash.par", hash_str], 
                      stdout=subprocess.PIPE)

The return value (a list) prints to stdout, but isn't captured

[4460475, 4406612, 4379510]

I've tried following it up with

value = p.communicate()[0]
value

..but value is an empty string, not the list of ints I was expecting, and which is being printed to stdout

I experimented with the solutions from Store output of subprocess.Popen call in a string but haven't been able to capture the output.

UPDATE:

stderr doesn't seem to yield anything either...and the list I'm looking for is being printed out...just not having any luck in capturing it. See below:

>>> p = subprocess.Popen(["/home/jfry/tools/search_by_hash.par", hash_str], 
                          stdout=subprocess.PIPE)
>>> 
[4460475, 4406612, 4379510]
    value, err  = p.communicate()
>>> value
''
>>> err

Thanks!

Are you trying to capture the data being sent to the subprocess's stdout, or are you trying to capture the subprocess's exit status? — Adam Rosenfield
– Adam Rosenfield, Commented Jun 5, 2011 at 21:51
Jakob - thanks, that was a typo in my question, but not at the prompt. I've changed the question. — Jeff Fry
– Jeff Fry, Commented Jun 5, 2011 at 21:53
Adam - I want the return value, in this case a list of ints, e.g. [4460475, 4406612, 4379510] — Jeff Fry
– Jeff Fry, Commented Jun 5, 2011 at 21:53
add stderr=subprocess.PIPE, just like you already have for stdout — snapshoe
– snapshoe, Commented Jun 5, 2011 at 22:12

Zach Kelling · Accepted Answer · 2011-06-05 22:00:22Z

5

Try checking stderr with p.communicate()[1].

edited Jun 5, 2011 at 22:00

answered Jun 5, 2011 at 21:51

Zach Kelling

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4 Comments

Jeff Fry Over a year ago

Zeekay, yes, I want the output of the command -- what's being printed to the screen after I type the command. Unfortunately, neither p.stdout.read() nor p.communicate()[0] is successfully capturing the output. Instead, both give me an empty string.

Zach Kelling Over a year ago

@Jeff Try checking stderr, `p.communicate()[1] for some clue as to what's going on.

Jeff Fry Over a year ago

Good suggestion, but no dice. I updated the question with the exact results from the python prompt. Thanks.

Jeff Fry Over a year ago

OK, with mita's suggestion above to include "stderr=subprocess.PIPE" in my Popen, I'm now able to see stderr with p.communicate()[1]. Thanks!

ead · Accepted Answer · 2016-08-21 15:01:14Z

Because the accepted answer does not solve the issue (at least for me):

To capture the stderr-output, the following should be done:

import subprocess
p = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)        
output, err = p.communicate()

The crucial part is the parameter stderr=subprocess.PIPE, without this parameter the stderr-output will not be captured and err will be None.

If you are interested only in the stderr-output you actually may use:

p = subprocess.Popen(command, stderr=subprocess.PIPE)        
err = p.communicate()[1]

In this case p.communicate()[0] is None.

user780363 · Accepted Answer · 2011-06-05 21:52:16Z

1

communicate is a method. So you should call it!

out = p.communicate()

answered Jun 5, 2011 at 21:52

user780363

1 Comment

Jeff Fry Over a year ago

Thanks, that was a typo in my question, but not at the prompt. I've changed the question.

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