Question

I am calling a python script with the following command line:

myscript.py --myopt="[(5.,5.),(-5.,-5.)]"

The question is — how to convert myopt to a list variable. My solution was to use optparse, treating myopt as a string, and using

(options, args) = parser.parse_args()    
myopt = eval(options.myopt)

Now, because I used eval() I feel a bit like Dobby the house elf, having knowingly transgressed the commandments of great (coding) wizards, and wanting to self-flagellate myself in punishment.

But are there better options for parsing lists or tuples or lists of tuples from the command line?.. I’ve seen solutions that use split(), but this won’t work here since this isn’t a simple list. Keep in mind, also, that this is being done in the context of mostly one-off scientific computing with no security concerns — so perhaps eval() isn’t as evil here?..

Answer 1

ast.literal_eval(node_or_string):

Safely evaluate an expression node or
a string containing a Python
expression. The string or node
provided may only consist of the
following Python literal structures:
strings, numbers, tuples, lists,
dicts, booleans, and None.

This can be used for safely evaluating
strings containing Python expressions
from untrusted sources without the
need to parse the values oneself.

So you can do

import ast
(options, args) = parser.parse_args()    
myopt = ast.literal_eval(options.myopt)