python USB (/dev/ttyACM0) serial terminal that works over ssh. reads from stdin, mingles the echo with whatever arrives from serial

verydumbterm.py

#!/usr/bin/env python3
import serial
from serial.serialutil import SerialException

import sys 
import select 
import tty 
import termios

def key_available():
    return select.select([sys.stdin], [], [], 0) == ([sys.stdin], [], [])


serial_port = serial.Serial(
    timeout=0.1,
    writeTimeout=2,
    port="/dev/ttyACM0"
)
if not serial_port.is_open:
    print("Exiting.  Could not open serial port:",serial_port.port)
    exit

old_tty_settings = termios.tcgetattr(sys.stdin)
old_port_attr = termios.tcgetattr(serial_port.fileno())
new_port_attr = termios.tcgetattr(serial_port.fileno())
new_port_attr[3] = new_port_attr[3] & ~termios.ECHO 
termios.tcdrain(serial_port.fileno())
termios.tcsetattr(serial_port.fileno(), termios.TCSADRAIN, new_port_attr)


try:
    tty.setcbreak(sys.stdin.fileno())

    serial_port.flushInput()
    serial_port.flushOutput()
    while serial_port.is_open:
        data_rx_bytes = serial_port.readline()
        data_rx_length = len(data_rx_bytes)
        if data_rx_length != 0:
            data_rx_str = str.rstrip(str(data_rx_bytes.decode('utf-8', 'strict')))
            print (data_rx_str)
        while key_available():
            c = sys.stdin.read(1)
            serial_port.write(c.encode('utf-8'))
            sys.stdout.write(c)
except SerialException:
    print("Serial Port closed. terminate.")
except KeyboardInterrupt:
    print("Break received,  Serial Port closing.")
finally:
    termios.tcsetattr(serial_port.fileno(), termios.TCSADRAIN, old_port_attr)
    termios.tcsetattr(sys.stdin, termios.TCSADRAIN, old_tty_settings)

Shout out to https://www.darkcoding.net/software/non-blocking-console-io-is-not-possible/