Subprocess in Python giving error

B Akshay · 02-07-2013, 06:09 AM

Hello,
I am a beginner in Python scripting. I tried run/executing two programs simultaneously and it gave following error after printing few output lines.

Code:

Starting with the Basic code

a =  -2

b =  0.0

c =  0.0

total =  0

1360238044.98

a =  -2.0

b =  1.02

c =  0.51

total =  0.0

1360238048.01

Traceback (most recent call last):
  File "C:/AKS/Subprocess/ex_prog01.py", line 43, in <module>
    root = Myprog()
  File "C:/AKS/Subprocess/ex_prog01.py", line 9, in __init__
    self.Task_03()
  File "C:/AKS/Subprocess/ex_prog01.py", line 38, in Task_03
    q = subprocess.Popen(self.Task_02())
  File "C:\Python27\lib\subprocess.py", line 679, in __init__
    errread, errwrite)
  File "C:\Python27\lib\subprocess.py", line 852, in _execute_child
    args = list2cmdline(args)
  File "C:\Python27\lib\subprocess.py", line 580, in list2cmdline
    for arg in seq:
TypeError: 'NoneType' object is not iterable

i am using python 2.7 and is installed on windows 7 machine.
Following is the code which i have tried with

Code:

import subprocess, time


class Myprog:

    def __init__(self,master=None):
        self.Task_03()
       

    def Task_01(self):
        v1 = 'Hello there'
        for x in range(1):
            time.sleep(2)
            print "\n", v1
        

    def Task_02(self):
        a = 1
        b = 1
        c = 1
        for i in range(2):
            total = ( a + b ) * c / (a + b + c)
            a = total - 2
            c = i * 0.51
            b = 2 * c
            time.sleep(3)
            print "\na = ", a
            print "\nb = ", b
            print "\nc = ", c
            print "\ntotal = ", total
            print "\n", time.time()
            

    def Task_03(self):
        print "\nStarting with the Basic code"
        q = subprocess.Popen(self.Task_02())
        p = subprocess.Popen(self.Task_01())
        print "\n\t\t\t\tEnd!!!!!"


root = Myprog()
root.mainloop()

Thanks in advance................

pan64 · 02-07-2013, 08:04 AM

class subprocess.Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)

... The arguments to Popen are as follows.

args should be a sequence of program arguments or else a single string. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.

in your case args is not a string and even, args is not iterable. that's why you got that error message. I do not understand how is it executed...

David1357 · 02-07-2013, 03:27 PM

Quote:

Originally Posted by B Akshay

Code:

    def Task_03(self):
        print "\nStarting with the Basic code"
        q = subprocess.Popen(self.Task_02())
        p = subprocess.Popen(self.Task_01())
        print "\n\t\t\t\tEnd!!!!!"

Task_01 and Task_02 do not return strings or arrays, which is what subprocess.Popen is expecting.

I wrote this function to make it easy to call subprocess.Popen. It returns True or False plus the output of the command. It redirects stderr to stdout so that you can display error output if something goes wrong. The important part is the call to shlex.split which converts the string into an array.

Code:

def command(_cmd_):
    """
    Execute a command and return the results
    """
    # We have to use shlex.split because unless we specify "shell=True",
    # subprocess.call requires the command to be split into an array of
    # arguments. We DO NOT use "shell=True" because it could be used to run
    # malicious code.
    args = shlex.split(_cmd_)
    p = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
    output, errors = p.communicate()
    if p.returncode < 0:
        output += "ERROR: Child was terminated by signal"
        return False, output
    else:
        if 0 != p.returncode:
            output += "ERROR: Child returned %d" % p.returncode
            return False, output

    return True, output

B Akshay · 02-08-2013, 01:23 AM

Thank you Pan64 for your inputs...........

David1357, i tried with the code given by you in my program, but still i am getting the following error,

Code:

Traceback (most recent call last):
  File "C:/AKS/Subprocess/ex_prog01.py", line 65, in <module>
    root = Myprog()
  File "C:/AKS/Subprocess/ex_prog01.py", line 9, in __init__
    self.Task_03()
  File "C:/AKS/Subprocess/ex_prog01.py", line 38, in Task_03
    p = subprocess.Popen(self.Task_02(), stdout=subprocess.PIPE,stderr=subprocess.STDOUT)
  File "C:\Python27\lib\subprocess.py", line 679, in __init__
    errread, errwrite)
  File "C:\Python27\lib\subprocess.py", line 852, in _execute_child
    args = list2cmdline(args)
  File "C:\Python27\lib\subprocess.py", line 580, in list2cmdline
    for arg in seq:
TypeError: 'NoneType' object is not iterable
>>>

the modification in the code which i have done is as follows, (guess its correct)

Code:

def Task_03(self):
        print "\nStarting with the Basic code"
        p = subprocess.Popen(self.Task_02(), stdout=subprocess.PIPE,stderr=subprocess.STDOUT)
        output, errors = p.communicate()
        if p.returncode < 0 :
            output += "ERROR: Child was terminated by signal"
            return False, output
        else :
            if 0 != p.returncode:
                output += "ERROR: Child returned %d"
                return False, output

        return True, output

        
        q = subprocess.Popen(self.Task_01(), stdout=subprocess.PIPE,stderr=subprocess.STDOUT)
        output1, errors1 = q.communicate()
        if q.returncode < 0 :
            output1 += "ERROR: Child was terminated by signal"
            return False, output1
        else:
            if 0 != q.returncode:
                output1 += "ERROR: Child returned %d"
                return False, Output1

        return True, output1
        print "\n\t\t\t\tEnd!!!!!"

please suggest some thing........................

B Akshay · 02-09-2013, 02:22 AM

my main purpose is to run to different processes in parallel..

Please suggest how to do same in Python..

Regards,
B Akshay

ntubski · 02-09-2013, 10:18 AM

Quote:

Originally Posted by B Akshay

my main purpose is to run to different processes in parallel..

I think you should learn what a process is first, it looks like maybe you actually want multiple threads.

pan64 · 02-09-2013, 11:37 AM

here is a simple tutorial about multithreading: http://www.tutorialspoint.com/python...ithreading.htm
and another one about multiprocessing: http://www.doughellmann.com/PyMOTW/m...ng/basics.html

David1357 · 02-14-2013, 10:07 AM

Quote:

Originally Posted by B Akshay

David1357, i tried with the code given by you in my program, but still i am getting the following error,
please suggest some thing........................

As I said before, Popen expects either a string or an array of strings.

Your functions do not return anything, much less strings.

It really looks like you want to use the Python thread library. This will allow you to pass your functions as arguments. Remember to pass the function name, not the function result:

Code:

try:
   thread.start_new_thread(self.Task_01)
   thread.start_new_thread(self.Task_02)
except:
   print "Error: unable to start threads"