How to Pass a String as stdin to subprocess.Popen?

Passing String Input to subprocess.Popen

The error encountered while attempting to pass a StringIO object as stdin to subprocess.Popen reveals that StringIO objects are not recognized as valid file-like objects by subprocess. To resolve this issue and successfully pass a string as stdin, it is necessary to first create a pipeline to the Popen process.

Using Explicit Pipeline Creation

The subprocess.communicate() documentation suggests creating a pipeline with stdin=PIPE to send data to the process's stdin. This can be achieved by modifying the code as follows:

import subprocess
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())

This code creates a pipe to the grep process, allowing the input string to be passed through the stdin argument.

Using subprocess.run (Python 3.5 )

For Python versions 3.5 and higher, subprocess.run can be used to pass input as a string and retrieve the output as a string in a single call:

#!/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)
print(p.stdout)

This approach provides a concise alternative to the explicit pipeline creation method.

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