No output from child Python executed with subprocess

Question

This is a simple command that works from command line:

> python -c "print('asdasd')"
asdasd

But it fails to output anything when executed from the script:

import os
import sys
import subprocess

cmd = [sys.executable] + ['-c', '"print(\'asdasd\')"']
print cmd
kwargs = {
  'stdout': subprocess.PIPE,
  'stderr': subprocess.PIPE,
  'stdin': subprocess.PIPE,
}

print subprocess.Popen(cmd).communicate()
print subprocess.Popen(cmd, **kwargs).communicate()

The output:

['C:\\Python27\\python.exe', '-c', '"print(\'asdasd\')"']
(None, None)
('', '')

Why it doesn't produce anything? I am out of ideas. Looks like a bug in Python 2.7.11 to me.

payet_s · Accepted Answer · 2016-06-20 11:13:40Z

1

First, you don't need those extra double-quotes for '"print(\'asdasd\')"' in your command line arguments. As it is, your code will just execute a python code that do the following: "print(\'asdasd\')". In other word, it will produce the string: print(\'asdasd\')

Obviously, creating a string won't print anything. Here is a patched version of your code:

import os
import sys
import subprocess

cmd = [sys.executable] + ['-c', 'print(\'asdasd\')']
print cmd
kwargs = {
    'stdout': subprocess.PIPE,
    'stderr': subprocess.PIPE,
    'stdin': subprocess.PIPE,
}

print subprocess.Popen(cmd).communicate()
print subprocess.Popen(cmd, **kwargs).communicate()

You may want to use the shlex module to parse your command line argument.

For example, using the shlex module and the triple quotes with a string format (do not forget quotes for the python path, otherwise characters such as backslash will be interpreted):

import sys
import shlex
import subprocess

cmd_str = '''"{}" -c "print('asdasd')"'''.format(sys.executable)
print(cmd_str)
cmd = shlex.split(cmd_str)
print(cmd)
kwargs = {
    'stdout': subprocess.PIPE,
    'stderr': subprocess.PIPE,
    'stdin': subprocess.PIPE,
}
print(subprocess.Popen(cmd).communicate())
print(subprocess.Popen(cmd, **kwargs).communicate())

answered Jun 20, 2016 at 11:13

payet_s

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1 Comment

anatoly techtonik Over a year ago

Thanks! I knew there is something obvious, but I thought it is somewhere in subprocess. I quoted the string, because it was quoted in raw command line, but when arguments are expressed in a list, Python quotes them itself.

j4hangir · Accepted Answer · 2016-06-20 11:18:45Z

0

Try this:

subprocess.Popen(['{} -c "print(\'ECHO\')"'.format(sys.executable)], shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()

edited Jun 20, 2016 at 11:18

answered Jun 20, 2016 at 11:13

j4hangir

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1 Comment

j4hangir Over a year ago

@anatolytechtonik Edited, give it another try.

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