How to run a command line from withing a python program

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January 25th, 2008, 03:20 PM

ironroot

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How to run a command line from withing a python program

Hi all
I have been working on this program for a few days and got most of the stuff to work so far but I am stuck on the last part which is I need to call a command line from with in my python program

Code:

import re

import os

test = "#define MEV"

times = range(1)



for i in times:

	runNum = str(i)

	run = "#define MEV " + runNum

	outfile=open('Temp.cpp', 'w')

	infile = open('Experiment2.1.cpp', 'r')

	

	for line in infile:

	 	if re.search(test, line):

			outfile.write(run)

			outfile.write("\n")

			print"here"



		else:

			outfile.write(line)

	os.system(g++ -lm -O3 outfile OMEC.cpp RandomOMEC.cpp -o evolve)
	os.system(evolve)

The only problem I am having is getting the last two lines to work. I need to call g++ compiler and the execute the evolve file that it makes any help would be great

January 26th, 2008, 07:07 AM

Arby

Contributing User

Join Date: May 2006

Location: North Yorkshire, England

Posts: 267

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Hi,

what error message do you get when you try to run the code? At first glance it looks like you are missing some quotes from the os.system call. For example:

Code:

>>> os.system(g++)
  File "<stdin>", line 1
    os.system(g++)
                 ^
SyntaxError: invalid syntax

whereas if I add double quotes

Code:

>>> os.system("g++")
g++: no input files
256

Presumably if I added your full command string in the quotes it should work but I know nothing about C. Therefore I don't know the proper way to execute a C program but changing os.system(evolve) to os.system("evolve") might be worth a try.

Also it's probably worth looking to see if there are some proper bindings for using C in python.

Hope that helps
Arby

February 18th, 2008, 11:37 AM

crescere

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Location: Wisconsin

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Just use this:

import os
os.system('system command you want to run')

February 18th, 2008, 03:29 PM

Schol-R-LEA

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Dev Shed Intermediate (1500 - 1999 posts)

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Just to clarify the issue: the argument for os.system() is a string, which is used as a shell command. The way you had written it, it was trying to pass it the sum of non-existent variables named 'g', 'lm', etc.

If you need to pass the value of a variable to the system command, you need to convert the value to a string and concatenate it to the command string:

Code:

    os.system('g++ -lm -O3 ' +  outfile.name + \ 
              ' OMEC.cpp RandomOMEC.cpp -o evolve');

Note that I passed it the name of the file, not the file object itself; the shell knows nothing about files or Python objects, and even if it did, trying to concatenate a string with a file handle makes about as much sense trying to add the number three to the color blue.

You could also use formatted string interpolation if it is a complicated string:

Code:

    os.system('g++ -lm -O3 %(file)s OMEC.cpp RandomOMEC.cpp -o evolve' \
              % {'file': outfile.name});

but that would probably be overkill for a single insertion.

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Last edited by Schol-R-LEA : February 18th, 2008 at 03:32 PM.