How to Correctly Pass Strings to `subprocess.Popen` via stdin?

Passing Strings to subprocess.Popen via stdin

Problem

Attempting to utilize StringIO with subprocess.Popen for stdin results in an AttributeError.

import subprocess
from cStringIO import StringIO
subprocess.Popen(['grep', 'f'], stdout=subprocess.PIPE, stdin=StringIO('one\ntwo\nthree\nfour\nfive\nsix\n')).communicate()[0]

Error

Traceback highlights the incompatibility of cStringIO.StringIO with the fileno() method required by subprocess.Popen.

AttributeError: 'cStringIO.StringI' object has no attribute 'fileno'

Solution

The subprocess.Popen documentation requires using stdin=PIPE for data transmission to stdin. The os.popen() replacement syntax is recommended:

pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin

Example

Adapting the original code:

from subprocess import Popen, PIPE, STDOUT

p = Popen(['grep', 'f'], stdout=PIPE, stdin=PIPE, stderr=STDOUT)    
grep_stdout = p.communicate(input=b'one\ntwo\nthree\nfour\nfive\nsix\n')[0]
print(grep_stdout.decode())
# -> four
# -> five
# ->

Python 3.5 Solution

For Python 3.5 :

#!/usr/bin/env python3
from subprocess import run, PIPE

p = run(['grep', 'f'], stdout=PIPE,
        input='one\ntwo\nthree\nfour\nfive\nsix\n', encoding='ascii')
print(p.returncode)
# -> 0
print(p.stdout)
# -> four
# -> five
# -> 

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