Inter-process communication using stream sockets (i.e. TCP).

tcp.py

import os
import socket

# `fork` current process. The child's PID is returned to the parent and `0` is
# returned to the child.
pid = os.fork()

# Create a socket. `AF_INET` is the address family for IPv4 and `SOCK_STREAM`
# is a connection-based protocol (i.e. TCP).
tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

# `gethostname` returns the current host name.
address = (socket.gethostname(), 5000)

if pid == 0:
    # Connect to the parent.
    tcp.connect(address)

    # The child process encodes the user input into `<class 'bytes'>` and sends
    # it to the parent.
    tcp.send(input().encode())
else:
    # The parent process binds `address` so it can receive data sent to it.
    tcp.bind(address)

    # Enable server to accept a maximum of one connection.
    tcp.listen(1)

    # Wait for an incoming connection. The return value is a tuple containing a
    # socket object representing the connection and the address of the incoming
    # connection.
    connection, address = tcp.accept()

    # Receive data from the connection. Note that `socket.socket.recv` by
    # default blocks, so the parent waits until the user has finished entering
    # the input.
    print(connection.recv(4096).decode())

    connection.close()

tcp.close()