subprocess: unexpected keyword argument capture_output

Question

When executing subprocess.run() as given in the Python docs, I get a TypeError:

>>> import subprocess
>>> subprocess.run(["ls", "-l", "/dev/null"], capture_output=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/subprocess.py", line 403, in run
    with Popen(*popenargs, **kwargs) as process:
TypeError: __init__() got an unexpected keyword argument 'capture_output'

I am running Python 3.6.6:

$ python3 --version
Python 3.6.6

`capture_output` is new in Python 3.7. – Warren Weckesser Nov 08 '18 at 13:53 — Warren Weckesser, Nov 08 '18 at 13:53

score 171 · Accepted Answer · edited Nov 06 '20 at 14:37

171

You inspected the wrong documentation, for python-3.6 this parameter does not exist, as can be found in the documentation (you select the version at the top left):

subprocess.run(args, *, stdin=None, input=None, stdout=None, stderr=None,
               shell=False, cwd=None, timeout=None, check=False, encoding=None,
               errors=None, env=None)

You can however easily "emulate" this by setting both stdout and stderr to PIPE:

from subprocess import PIPE

subprocess.run(["ls", "-l", "/dev/null"], stdout=PIPE, stderr=PIPE)

In fact, if we look at the source code of the python-3.7 version, where the feature was introduced, we see in the source code [GitHub]:

if capture_output:
    if ('stdout' in kwargs) or ('stderr' in kwargs):
        raise ValueError('stdout and stderr arguments may not be used '
                         'with capture_output.')
    kwargs['stdout'] = PIPE
    kwargs['stderr'] = PIPE

edited Nov 06 '20 at 14:37

Nordle

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answered Nov 08 '18 at 13:56

Willem Van Onsem

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1

I get "name 'PIPE' is not defined" if I try to use the solution provided here. – Scorb Feb 19 '20 at 01:34
2

@ScottF: you did import it `from subprocess import PIPE` (as you can see at the top of the code fragment)? – Willem Van Onsem Feb 19 '20 at 06:42
3

you could also just use `stdout=subprocess.PIPE` to the same effect without the import – RufusVS Jul 14 '21 at 23:50

score 8 · Answer 2 · answered Nov 06 '19 at 09:16

8

The simplest method is to use the subprocess.check_output function:

import subprocess
subprocess.check_output(["ls", "-l", "/dev/null"])

answered Nov 06 '19 at 09:16

Hugo Sohm

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score 3 · Answer 3 · answered Sep 11 '19 at 18:32

3

I ran into this error because I was calling subprocess.call (which is the old high level API) instead of subprocess.run.

answered Sep 11 '19 at 18:32

Boris Verkhovskiy

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subprocess: unexpected keyword argument capture_output

3 Answers3

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