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?

Answer 1

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.

Answer 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()

Answer 3

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