Multiprocessing with subprocessing functions

Question

I'm writing a program that has functions that use subprocesses to receieve/transmit messages however, I want to be able to run the receive in the background whilst being able to print whatever is being received.

Please see code:

#!/usr/bin/python

import subprocess,time, timeit
from multiprocessing import Process, Queue
import re, os, pprint, math
from collections import defaultdict

Dict = {}
identifier = ""
hexbits = []
count = defaultdict(int)


def receive():
    process = subprocess.Popen('receivetest -f=/dev/pcan32', stdout=subprocess.PIPE)
    lines = iter(process.stdout.readline, "")
    try:
        start = time.clock()
        for line in lines:
            if re.match(r"^\d+.*$",line):
                splitline = line.split()
                del splitline[1:4]
                identifier = splitline[1]
                count[identifier] += 1
                end = time.clock()
                timing = round((end - start) * 10000, 100)
                dlc = splitline[2]
                hexbits = splitline[3:]
                Dict[identifier] = [dlc, hexbits, count[identifier],int(timing)]
                time.sleep(0.005)
                start = end 


    except keyboardinterrupt:
        pass

def transmit(command):
    transfile = "transfile.txt"
    file = open(transfile,'w')
    file.write("# please type your can message below\n")
    file.write("# using the following example\n")
    file.write("# m s [can id] [data bytes]\n")
    file.close()
    os.system("vi transfile.txt")
    trans = subprocess.Popen(command, stdout=subprocess.pipe)
    lines = trans.communicate()
    print lines

operation = raw_input('please enter T for transmit and R for receive: ')

if((operation == 'T') or (operation == 't')):
    transmit(["transmitest", "transfile.txt","-f=/dev/pcan32"])
elif((operation == 'R') or (operation == 'r')):
    if __name__ == '__main__':
        os.system("echo \"i 0x011c e\"  >/dev/pcan32")
        #receive(["receivetest", "-f=/dev/pcan32"])
        run_receive = Process(target=receive, args=())
        run_receive.start()
        run_receive.join()
        for identifier, hexbits in Dict.items():
            os.system("clear")
            pprint.pprint(Dict, width = 150)

else:
    print("PLEASE ENTER VALID OPTION")

However I am getting the following errors:

Trace (most recent call last):
    File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
        self.run()
    File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in run
        self._target(*self._args, **self._kwargs)
    File "./cancheck.py", line 16, in receive
        process = subprocess.Popen(command, stdout=subprocess.PIPE)
    File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
        errread, errwrite)
    File "/usr/lib/python2.7/subprocess.py", line 679, in _execute_child
        raise child_exception
OSError: [Errno 2] No such file or directory

Could someone please help, I'm not sure how to multiprocess a function that contains subprocesses.

Answer 1

It looks like the subprocess command invocation can't resolve a file. I'd recommend verifying that transmitest is in the execution path, and that transfile.txt and /dev/pcan32 all exist.

As another note, in the receive definition, your Popen invocation looks like it needs to be changed.

# Original version
# process = subprocess.Popen('receivetest -f=/dev/pcan32', stdout=subprocess.PIPE)

# Change to this:
process = subprocess.Popen('receivetest -f=/dev/pcan32'.split(), stdout=subprocess.PIPE)

The reason is that Popen is expecting an iterable sequence as its first parameter.