How Can I Pipe a String to subprocess.Popen's stdin?

How to Pipe a String into subprocess.Popen via the Stdin Argument

The subprocess.Popen function enables Python scripts to spawn new processes and interact with their standard inputs, outputs, and errors. One common use case involves passing data to the child process via its stdin, which can be achieved by specifying stdin=PIPE during object creation.

However, attempting to use a cStringIO.StringIO object as the stdin stream results in an error, as it lacks the fileno attribute expected by subprocess.Popen. To resolve this issue, utilize the following techniques:

Using Popen.communicate()

As per the official documentation, Popen.communicate() expects data to be passed through the input parameter, rather than the stdin stream. Modify your code to:

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())

Using subprocess.run (Python 3.5 )

For Python versions 3.5 and above, subprocess.run provides a simplified way to manage subprocesses. It allows you to pass string input and obtain string output in a single function 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)

By adopting these approaches, you can effectively pass a string into subprocess.Popen via the stdin argument, enabling your programs to seamlessly interact with external processes.

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