specifying a list as a command line argument in python

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Question :

specifying a list as a command line argument in python

I am using getopt to process a command line optional argument, which should accept a list. Something like this:

foo.py --my_list=[1, 2, 3, 4,5] 

But this trims everything after “[1,”

My questions are:
A) Is there a way to specify a list without converting it into a string? (using getopt)

B) If I am to convert the list into a string, how to convert this list to a string? e.g. something like mylist.split(“?”) to get rid of square brackets ?? is there a better way?

Thank you

Asked By: cppb


Answer #1:

There are two options that I can think of:

  • Use optparse, and use append action to specify what you want to do as: foo.py --my_list=1 --my_list=2 ....
  • Specify your commandline as foo.py --my_list='1,2,3,4,5', and then use x.split(',') to get your values in a list. You can use getopt or optparse for this method.

The advantage of the first method is that you can get integer values in the list directly, at the expense of the commandline being longer (but you can add a single-charecter option for --my_list if you want). The advantage of the second is shorter command line, but after the split(), you need to convert the string values '1', '2', etc., to integers (pretty easy as well).

Answered By: Alok Singhal

Answer #2:

If I can’t use a standard parser (optparse or argparse) to my application then I use the ast.literal_eval function to parse input arguments of type list as follows:

import sys, ast

inputList = ast.literal_eval( sys.argv[1] )
print type( inputList )
print inputList

Let suppose that this code is stored in testParser.py file. By executing the script:

$ python testParser.py  "[1,2,3,4, [123, 456, 789], 'asdasd']"

we get the following output:

<type 'list'>
[1, 2, 3, 4, [123, 456, 789], 'asdasd']

So, using the secure enough ast.literal_eval function and inserting the list as a string of code we have the desirable result.

Useful links:

Using python’s eval() vs. ast.literal_eval()?


Answered By: funk

Answer #3:

Maybe you should just enclose the argument in quotes?

foo.py "--my_list=[1, 2, 3, 4,5]"

Otherwise every space will be treated as a separator for arguments.

Answered By: Joey

Answer #4:

An updated way is to use the argparse library and add the list as a command line argument. This way, you don’t have to do any manual parsing.


parser = argparse.ArgumentParser()
        nargs="*",  # expects ? 0 arguments
        default=[35, 40, 50, 60, 70, 80, 90],  # default list if no arg value

Which you would then call like this:

python3 someprogram.py -values 1 2 3
Answered By: Nick Predey

Answer #5:

From the python optparse help page:

parser.add_option("-p", type="float", nargs=3, dest="point")

As it parses the command line

-f foo.txt -p 1 -3.5 4 -fbar.txt

optparse will set

options.f = "foo.txt"
options.point = (1.0, -3.5, 4.0)
options.f = "bar.txt"

Answered By: highvelcty

Answer #6:

I have been redirected from the dup question:
How to pass an array to python through command line – Stack Overflow

I’m answering that question:
The arguments on the command line are strings, they’re not parsed like literals in the program.
argv construct the strings automatically to a list from command line arguments (as separated by spaces), in short,
sys.argv is a list.

    from sys import argv
    cmd = "arr = {sys.argv[1]}".format(argv)
Answered By: Calculus

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