Virtual Serial Device in Python?

I know that I can use e.g. pySerial to talk to serial devices, but what if I don't have a device right now but still need to write a client for it? How can I write a "virtual serial device" in Python and have pySerial talk to it, like I would, say, run a local web server? Maybe I'm just not searching well, but I've been unable to find any information on this topic.

this is something I did and worked out for me so far:

import os, pty, serial

master, slave = pty.openpty()
s_name = os.ttyname(slave)

ser = serial.Serial(s_name)

# To Write to the device
ser.write('Your text')

# To read from the device
os.read(master,1000)

If you create more virtual ports you will have no problems as the different masters get different file descriptors even if they have the same name.

I was able to emulate an arbitrary serial port ./foo using this code:

SerialEmulator.py

import os, subprocess, serial, time

# this script lets you emulate a serial device
# the client program should use the serial port file specifed by client_port

# if the port is a location that the user can't access (ex: /dev/ttyUSB0 often),
# sudo is required

class SerialEmulator(object):
    def __init__(self, device_port='./ttydevice', client_port='./ttyclient'):
        self.device_port = device_port
        self.client_port = client_port
        cmd=['/usr/bin/socat','-d','-d','PTY,link=%s,raw,echo=0' %
                self.device_port, 'PTY,link=%s,raw,echo=0' % self.client_port]
        self.proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
        time.sleep(1)
        self.serial = serial.Serial(self.device_port, 9600, rtscts=True, dsrdtr=True)
        self.err = ''
        self.out = ''

    def write(self, out):
        self.serial.write(out)

    def read(self):
        line = ''
        while self.serial.inWaiting() > 0:
            line += self.serial.read(1)
        print line

    def __del__(self):
        self.stop()

    def stop(self):
        self.proc.kill()
        self.out, self.err = self.proc.communicate()

socat needs to be installed (sudo apt-get install socat), as well as the pyserial python package (pip install pyserial).

Open the python interpreter and import SerialEmulator:

>>> from SerialEmulator import SerialEmulator
>>> emulator = SerialEmulator('./ttydevice','./ttyclient') 
>>> emulator.write('foo')
>>> emulator.read()

Your client program can then wrap ./ttyclient with pyserial, creating the virtual serial port. You could also make client_port /dev/ttyUSB0 or similar if you can't modify client code, but might need sudo.

Also be aware of this issue: https://stackoverflow.com/questions/34831131/pyserial-does-not-play-well-with-virtual-port

It may be easier to using something like com0com (if you're on Windows) to set up a virtual serial port, and develop on that.

If you are running Linux you can use the socat command for this, like so:

socat -d -d pty,raw,echo=0 pty,raw,echo=0

When the command runs, it will inform you of which serial ports it has created. On my machine this looks like:

2014/04/23 15:47:49 socat[31711] N PTY is /dev/pts/12
2014/04/23 15:47:49 socat[31711] N PTY is /dev/pts/13
2014/04/23 15:47:49 socat[31711] N starting data transfer loop with FDs [3,3] and [5,5]

Now I can write to /dev/pts/13 and receive on /dev/pts/12, and vice versa.

Maybe a loop device will do the job if you need to test your application without access to a device. It's included in pySerial 2.5 https://pythonhosted.org/pyserial/url_handlers.html#loop

It depends a bit on what you're trying to accomplish now...

You could wrap access to the serial port in a class and write an implementation to use socket I/O or file I/O. Then write your serial I/O class to use the same interface and plug it in when the device is available. (This is actually a good design for testing functionality without requiring external hardware.)

Or, if you are going to use the serial port for a command line interface, you could use stdin/stdout.

Or, there's this other answer about virtual serial devices for linux.