39

I want to invoke a script, piping the contents of a string to its stdin and retrieving its stdout.

I don't want to touch the real filesystem so I can't create real temporary files for it.

using subprocess.check_output I can get whatever the script writes; how can I get the input string into its stdin though?

subprocess.check_output([script_name,"-"],stdin="this is some input") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python2.7/subprocess.py", line 537, in check_output process = Popen(stdout=PIPE, *popenargs, **kwargs) File "/usr/lib/python2.7/subprocess.py", line 672, in __init__ errread, errwrite) = self._get_handles(stdin, stdout, stderr) File "/usr/lib/python2.7/subprocess.py", line 1043, in _get_handles p2cread = stdin.fileno() AttributeError: 'str' object has no attribute 'fileno'
Improve this question
3
  • 3
    The stdin arg for check_output() must be a file object, not a string. Commented Apr 11, 2012 at 10:00
  • @jdi obviously; so how to give it something that quacks like a file but isn't a file? Commented Apr 11, 2012 at 10:06
  • By choosing @larsmans answer :-) its a lot easier if you don't try to overuse the check_output convenience function and just do a normal Popen + communicate. Otherwise you would be expected to open your own pipe manually before hand, and then pass that to the check_output(stdin) and then write to it. Commented Apr 11, 2012 at 10:09

3 Answers 3

Reset to default
37

Use Popen.communicate instead of subprocess.check_output.

from subprocess import Popen, PIPE p = Popen([script_name, "-"], stdin=PIPE, stdout=PIPE, stderr=PIPE) stdout, stderr = p.communicate("this is some input")
Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

This depends on the tool. Some are designed to read standard input if you call them without arguments, others want an explicit dash (and many will accept a dash as standard input if they accept multiple file name arguments).
This is not really correct any longer; in Python 3.5+ you can do this easily with subprocess.run()
31

In Python 3.4 and newer, you can use the input keyword parameter to send input via STDIN when using subprocess.check_output()

Quoting from the standard library documentation for subprocess.check_output():

The input argument is passed to Popen.communicate() and thus to the subprocess’s stdin. If used it must be a byte sequence, or a string if universal_newlines=True. When used, the internal Popen object is automatically created with stdin=PIPE, and the stdin argument may not be used as well.

Example:

>>> subprocess.check_output(["sed", "-e", "s/foo/bar/"], ... input=b"when in the course of fooman events\n") b'when in the course of barman events\n' >>> >>> # To send and receive strings instead of bytes, >>> # pass in universal_newlines=True >>> subprocess.check_output(["sed", "-e", "s/foo/bar/"], ... universal_newlines=True, ... input="when in the course of fooman events\n") 'when in the course of barman events\n'
Improve this answer

1 Comment

Is it possible to pass input to subprocess.check_output() as demonstrated here in python 2.7? I know that the input option is not able to be used but subprocess.communicate(...) is not passing my input as expected and I seem to have no other option.
6

Here's a check_output backported version for python 2.7 with input.

from subprocess import (PIPE, Popen, CalledProcessError) def check_output_input(*popenargs, **kwargs): """Run command with arguments and return its output as a byte string. If the exit code was non-zero it raises a CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute and output in the output attribute. The arguments are the same as for the Popen constructor. Example: >>> check_output(["ls", "-l", "/dev/null"]) 'crw-rw-rw- 1 root root 1, 3 Oct 18 2007 /dev/null\n' The stdout argument is not allowed as it is used internally. To capture standard error in the result, use stderr=STDOUT. >>> check_output(["/bin/sh", "-c", ... "ls -l non_existent_file ; exit 0"], ... stderr=STDOUT) 'ls: non_existent_file: No such file or directory\n' There is an additional optional argument, "input", allowing you to pass a string to the subprocess's stdin. If you use this argument you may not also use the Popen constructor's "stdin" argument, as it too will be used internally. Example: >>> check_output(["sed", "-e", "s/foo/bar/"], ... input=b"when in the course of fooman events\n") b'when in the course of barman events\n' If universal_newlines=True is passed, the return value will be a string rather than bytes. """ if 'stdout' in kwargs: raise ValueError('stdout argument not allowed, it will be overridden.') if 'input' in kwargs: if 'stdin' in kwargs: raise ValueError('stdin and input arguments may not both be used.') inputdata = kwargs['input'] del kwargs['input'] kwargs['stdin'] = PIPE else: inputdata = None process = Popen(*popenargs, stdout=PIPE, **kwargs) try: output, unused_err = process.communicate(inputdata) except: process.kill() process.wait() raise retcode = process.poll() if retcode: cmd = kwargs.get("args") if cmd is None: cmd = popenargs[0] raise CalledProcessError(retcode, cmd, output=output) return output
Improve this answer

1 Comment

it would be great if this could get stderr too!

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.