Receive data from host computer using Circuit Python on Circuit Playground Express - adafruit

Question

I am using a Circuit Playground Express from Adafruit, and I'm programming it with Circuit Python.
I want to read data transmitted from the computer to which the Circuit Playground Express is connected via USB. Using input() works fine, but I would rather get the buffer of the serial instead, so that the loop would go on while there's no input. Something like serial.read().
import serial does not work on Circuit Python, or maybe I must install something. Is there anything else I could do to read the serial buffer using Circuit Python?

Answer 1

Aiden's answer lead me in the right direction and I found a nice (and slightly different) example of how to use supervisor.runtime.serial_bytes_available from Adafruit here (specifically lines 89-95): https://learn.adafruit.com/AT-Hand-Raiser/circuitpython-code
A minimum working example for code.py that takes the input and formats a new string in the form "RX: string" is
import supervisor
print("listening...")
while True:
if supervisor.runtime.serial_bytes_available:
value = input().strip()
# Sometimes Windows sends an extra (or missing) newline - ignore them
if value == "":
continue
print("RX: {}".format(value))
Tested on: Adafruit CircuitPython 4.1.0 on 2019-08-02; Adafruit ItsyBitsy M0 Express with samd21g18. Messages sent using the serial connection in mu-editor on macOS.
Sample output
main.py output:
listening...
hello!
RX: hello!

Answer 2

I got a simple example to work based on above posts, not sure how stable or useful it is, but still posting it here:
CircuitPython Code:
import supervisor
while True:
if supervisor.runtime.serial_bytes_available:
value = input().strip()
print(f"Received: {value}\r")
PC Code
import time
import serial
ser = serial.Serial('COM6', 115200) # open serial port
command = b'hello\n\r'
print(f"Sending Command: [{command}]")
ser.write(command) # write a string
ended = False
reply = b''
for _ in range(len(command)):
a = ser.read() # Read the loopback chars and ignore
while True:
a = ser.read()
if a== b'\r':
break
else:
reply += a
time.sleep(0.01)
print(f"Reply was: [{reply}]")
ser.close()
c:\circuitpythontest>python TEST1.PY
Sending Command: [b'hello\n\r']
Reply was: [b'Received: hello']

Answer 3

This is now somewhat possible!
In the January stable release of CircuitPython 3.1.2 the function serial_bytes_available was added to the supervisor module.
This allows you to poll the availability of serial bytes.
For example in the CircuitPython firmware (i.e. boot.py) a serial echo example would be:
import supervisor
def serial_read():
if supervisor.runtime.serial_bytes_available():
value = input()
print(value)
and ensure when you create the serial device object on the host side you set the timeout wait to be very small (i.e. 0.01).
i.e in python:
import serial
ser = serial.Serial(
'/dev/ttyACM0',
baudrate=115200,
timeout=0.01)
ser.write(b'HELLO from CircuitPython\n')
x = ser.readlines()
print("received: {}".format(x))