How to Correctly Pass String Input to `subprocess.Popen`'s stdin?

Passing String Input to Subprocess.Popen via Stdin

Problem:

Passing a string into the stdin argument of subprocess.Popen using a cStringIO.StringIO object results in an error, as the object lacks the necessary fileno attribute.

Solution:

To resolve this issue, it is recommended to use the more straightforward approach outlined in the Popen.communicate() documentation. By setting stdin=PIPE, you can create a pipe for stdin and provide the string input directly to the communicate method.

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())

# Output:
# -> four
# -> five
# ->

Additional Note:

For Python 3.5 (3.6 for encoding), subprocess.run simplifies the process by allowing you to pass string input and retrieve the output as a string in a single call.

from subprocess import run, PIPE

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

# Output:
# -> 0
print(p.stdout)

# Output:
# -> four
# -> five
# -> 

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