#! /usr/bin/env python3
# -*- coding: utf-8 -*-

import subprocess

# why is this not executed immediately?
lpr =  subprocess.Popen("/usr/bin/lpr",              # on raspbian: /usr/bin/lp
                        stdin=subprocess.PIPE,
                        stdout=subprocess.DEVNULL,   # proposed by user elias
                        close_fds=True)              # proposed by user elias

output = "Username: testusern".encode()
         + "Password: p4ssw0rdn".encode()

lpr.stdin.write(output)

while True:
    pass

#! /usr/bin/env python3
# -*- coding: utf-8 -*-

import subprocess
import time

# why is this not executed immediately?
lpr =  subprocess.Popen("/usr/bin/lpr",              # on raspbian: /usr/bin/lp
                        stdin=subprocess.PIPE,
                        stdout=subprocess.DEVNULL,   # proposed by user elias
                        close_fds=True)              # proposed by user elias

output = "Username: testusern".encode()
         + "Password: p4ssw0rdn".encode()

lpr.stdin.write(output)

time.sleep(20)