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!

Answer 1

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.

Answer 2

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

Answer 3

communicate is a method. So you should call it!

out = p.communicate()