Gilad Naor

Recent

Seattle and Trust
Moving to Seattle
Startup Investing for Everyone
There's a Choice

Python, Subprocess and Multiple Arguments

Calling external programs from within a python script is a pretty common task. However, most of the times when I want to pass some arguments to the external program, I usually get stuck. The official Python documentation for the subprocess module is a bit lacking, in my humble opinion, in its treatment of argument passing.

This is not to say that all the information isn’t available in the official documentation. In fact, it is. It’s just not where I always look for it. The first thing I look for are examples. I simply do not have time to read the entire documentation page for something which should not be too difficult.

The basic idea is this:

  1. Either provide a string of the entire command and argument, but use Shell=True
  2. Provide a string for only the command
  3. Provide a list of strings, where the first item is the command and the other items are the arguments

It’s the third option that keeps on giving me trouble, so here is a simple working example for reference:

 1 **#!/bin/env python**  
 2 import subprocess  
 3   
 4 program_name = "ls"  
 5 arguments = ["-l", "-a"]  
 6   
 7 command = [program_name]  
 8 command.extend(arguments)  
 9   
10 output = subprocess.Popen(command, stdout=subprocess.PIPE).communicate()[0]  
11 print output  

Passing Arguments with Quotes

If you want to pass arguments that are passed with quotes in the shell, then just pass them as a single list item, without the quotes.

  1. James Gardner wrote the most comprehensive introduction to the python subprocess module I’ve encountered.
  2. The official documentation.

Further Reading

  1. Bash: Passing Arguments with Quotes
  2. Why Not to Write Shell Scripts