Python Pyserial read data form multiple serial ports at same time
Question: I'm trying to read out multiple serial ports at the same time with Python 2.7
and PySerial.  
Features should be:

  1. in the main program I get all open serial ports, open them and append the serial object to serialobjects
  2. I want to read each serial port data in one subprocess for parallelization 

The big problem is: how do I pass the serial port object to the subprocess?  
OR:  
Does another (and maybe better) solution exist to this? (Maybe
[this](http://stackoverflow.com/questions/16255807/pyserial-is-there-a-way-to-
select-on-multiple-ports-at-once): How do I apply twisted serial ports to my
problem?)

## EDIT

I think I wasn't totally clear what i want to achieve.  
I want to read out 2 or more serial ports at the same time. Because of timeout
and readout times it isn't possible to read them out at the same time in one
process.  
The following approach

    
    
    ser1 = serial.Serial(port="COM1",baudrate=9600)
    ser2 = serial.Serial(port="COM2",baudrate=9600)
    
    ser1.write('command for reading out device 1')
    output1 = ser1.readline()
    
    ser2.write('command for reading out device 2')
    # now you have to wait at least 100ms for device 2 to respond
    output2 = ser2.readline()
    

doesn't serve my needs.

Another approch is to parallelize the serial readings in subprocesses.

## main.py

    
    
    import serial   # serial communication
    from subprocess import Popen, PIPE
    
    ports = ["COM1", "COM2"]
    for port in ports:
        ser = serial.Serial()
        ser.port=port
        ser.baudrate=9600
        # set parity and ...
    
        serialobjects.append(ser)
    
    # call subprocess
    # pass the serial object to subprocess
    # read out serial port
    
    
    # HOW TO PASS SERIAL OBJECT HERE to stdin
    p1 = Popen(['python', './ReadCOM.py'], stdin=PIPE, stdout=PIPE, stderr=PIPE) # read COM1 permanently
    p2 = Popen(['python', './ReadCOM.py'], stdin=PIPE, stdout=PIPE, stderr=PIPE) # read COM2 permanently
    
    for i in range(10):
        print "received from COM1: %s" % p1.stdout.readline() # print output from ReadCOM.py for COM1
        print "received from COM2: %s" % p2.stdout.readline() # print output from ReadCOM.py for COM2
    

## ReadCOM.py (taken from [related
post](http://stackoverflow.com/questions/8260502/python-multiple-threads-
multiple-processes-for-reading-serial-ports) and edited)

    
    
    import sys
    
    while True:  # The program never ends... will be killed when master is over.
        # sys.stdin.readline()
    
        ser.write('serial command here\n') # send command to serial port
        output = ser.readline() # read output
    
        sys.stdout.write(output) # write output to stdout
        sys.stdout.flush()
    

Thanks in advance!


Answer: First change `ReadCOM.py` to receive arguments

    
    
    import sys
    import serial
    
    ser = serial.Serial(port=sys.argv[1],baudrate=int(sys.argv[2]))
    while True:  # The program never ends... will be killed when master is over.
        # sys.stdin.readline()
    
        ser.write('serial command here\n') # send command to serial port
        output = ser.readline() # read output
    
        sys.stdout.write(output) # write output to stdout
        sys.stdout.flush()
    

and after pass it in `main.py`:

    
    
    from subprocess import Popen, PIPE
    
    # call subprocess
    # pass the serial object to subprocess
    # read out serial port
    
    
    # HOW TO PASS SERIAL OBJECT HERE to stdin
    p1 = Popen(['python', './ReadCOM.py', "COM1", "9600"], stdin=PIPE, stdout=PIPE, stderr=PIPE) # read COM1 permanently
    p2 = Popen(['python', './ReadCOM.py', "COM2", "9600"], stdin=PIPE, stdout=PIPE, stderr=PIPE) # read COM2 permanently
    
    for i in range(10):
        print "received from COM1: %s" % p1.stdout.readline() # print output from ReadCOM.py for COM1
        print "received from COM2: %s" % p2.stdout.readline() # print output from ReadCOM.py for COM2
    

