Optional Command Line Parameter in Python

Question

I am trying to retrieve optional command line parameters for a Python script (2.7 under Windows) and things are not going smoothly. The code is:

parser = argparse.ArgumentParser(description = 'Process display arguments')
parser.add_argument('-t', nargs = '?', default = 'baz')
args = parser.parse_args(['-t'])
print args.t

If I run "program.py" with no parameter, args.t is printed as None. If I run "program.py -t", args.t is printed as None. If I run "program.py -t foo", args.t is printed as None.

Why am I not getting the value from the command line into args.t?

Weeble · Accepted Answer · 2012-02-16 19:19:56Z

4

Don't pass ['-t'] to parse_args. Just do:

args = parser.parse_args()

Any arguments you pass to parse_args are used instead of your command-line. So with that argument it doesn't matter what command-line you use, argparse never sees it.

answered Feb 16, 2012 at 19:19

Weeble

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1 Comment

Count Boxer Over a year ago

And '-h' now works as well! I think I got confused when the examples moved from "print parser.parse_args([ '-t']" to actually using it in the script.

David Robinson · Accepted Answer · 2012-02-16 19:25:33Z

2

The line

args = parser.parse_args(["-t"])

is passing the command line arguments ["-t"] to the parser. You want to work with the actual command line arguments, so change the line to

args = parser.parse_args()

answered Feb 16, 2012 at 19:25

David Robinson

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Comments

unutbu · Accepted Answer · 2012-02-16 19:23:54Z

1

Use the const keyword argument:

import argparse
parser = argparse.ArgumentParser(description = 'Process display arguments')
parser.add_argument('-t', nargs = '?', const = 'baz', default = 'baz')
args = parser.parse_args()
print args.t

Running it yields:

% test.py          # handled by `default`
baz
% test.py -t       # handled by `const`
baz
% test.py -t blah
blah

answered Feb 16, 2012 at 19:23

unutbu

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1 Comment

jcollado Over a year ago

This is explained in the documentation: Note that for optional arguments, there is an additional case - the option string is present but not followed by a command-line arg. In this case the value from const will be produced.

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