How to write to the STDIN of another Python process?

Question

I have a Python program which code contains the following fragment:

def Main():
    pid = os.getpid()
    print 'Process id: {}'.format(pid)

then, in other part of the code it keeps listening on its STDIN.

This process outputs the following:

Process id: 3824

From another Python process I want to write to the STDIN of the process above given its PID.

My question is: what code I need to do that from the second process?

I'm using Windows.

@cᴏʟᴅsᴘᴇᴇᴅ, could you be a bit more specific?, thanks — Angel, Aug 13 '17 at 06:10
Look at the subprocess module and the ways to control input and output of the subprocess. Then ask a more precise question. — Serge Ballesta, Aug 13 '17 at 06:10
@CoryMadden, that example is for Linux and doesn't use the PID of the process. I need the code for Windows and using the PID of the process — Angel, Aug 13 '17 at 06:10

score 2 · Accepted Answer · answered Aug 13 '17 at 06:31

A program can only redirect the standard input and output of processes it creates("child processes"). To talk to another process using standard output, that process must already be listening to you.

It's the OS that controls what the file descriptors of a process refer to. That is, the stdin of your process will be determined by the os and the context in which the programs are executed. To allow interprocess communication, you could look into the facilities provided by the OS for that, for example using a shell script and the piping operator.

What you'd want is a named pipe, which is basically a file-like object which you can write and read from, and which act as a bridge between processes. Unfortunately, python doesn't seem to have "native" support for that on Windows. You need the python for windows extension for that.

Alternatively, you could use a socket.

Named pipes are supported by the [multiprocessing module](https://docs.python.org/3/library/multiprocessing.html#module-multiprocessing.connection) via `connection.Listener` and `connection.Client`. Other functions in multiprocessing implicitly use named pipes, but these let you explicitly set the name/address, e.g. `r'\\.\PIPE\spam'`. — Eryk Sun, Aug 13 '17 at 08:24
Answering the question as posed would involve calling `AttachConsole` and `WriteConsoleInput`, assuming the process is using a console for its standard input. But that's not a normal approach, especially if you have control of the source for both the server and the client. — Eryk Sun, Aug 13 '17 at 08:27

score 0 · Answer 2 · answered Aug 13 '17 at 06:56

0

Instead of writing to STDIN use Queue to communicate between processes from multiprocessing import Process, Queue

def f(q):
    q.put([42, None, 'hello'])

q = Queue()
p1 = Process(target=f, args=(q,))
p1 = Process(target=f, args=(q,))
# start and join

In another process do

print(q.get())    # prints "[42, None, 'hello']"

answered Aug 13 '17 at 06:56

naren

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Doesn't this way get penalized by GIL because `q` is shared? – Verthais Jun 14 '21 at 14:49

How to write to the STDIN of another Python process?

2 Answers2