How do I pass a string into subprocess.run using stdin?

Question

How do I pass a string "foo" to a program expecting it on stdin if I'm using Python's subprocess.run()?

Answer 1

Simplest possible example, send foo to cat and let it print to the screen:

import subprocess

subprocess.run(['cat'], input=b'foo\n')

Notice that you need to send binary data and a carriage return.