argparse parser stores lists instead of strings

[Andrew Berg]

I've set up groups of arguments for a script I'm writing, and any time I
give an argument a value, it gets stored as a list instead of a string,
even if I explicitly tell it to store a string. Arguments declared with
other types (e.g. float, int) and default values are stored as expected.
For example:

vidin_args=parser.add_argument_group('Video Input Options', 'Various
options that control how the video file is demuxed/decoded.')
vidin_args.add_argument('-m', dest='vmode', nargs=1, type=str,
metavar='video_mode', choices=['ntsc', 'pal', 'film', 'ivtc'],
default='ntsc', help='Valid values are "ntsc", "pal", "film" and "ivtc".')
....more arguments...
opts=parser.parse_args()

If I assign a value on the command line (e.g. -m pal), opts.vmode is a
list, otherwise it's a string. This is pretty bad since I can't know
whether to get opts.vmode or opts.vmode[0] later in the script. I could
loop through all the options and convert each option to a string, but
that's not really something I want to do, especially if I start adding
more options.

I'm pretty new to Python, and I might have missed something, but I have
been looking!
In case it matters, I'm learning Python 3.2 and have no intention of
using older code (once I have one version of Python covered, then I'll
look into making code that's compatible with 2.x if I have to).

 

[Peter Otten]

I've set up groups of arguments for a script I'm writing, and any time I
give an argument a value, it gets stored as a list instead of a string,
even if I explicitly tell it to store a string. Arguments declared with
other types (e.g. float, int) and default values are stored as expected.
For example:

vidin_args=parser.add_argument_group('Video Input Options', 'Various
options that control how the video file is demuxed/decoded.')
vidin_args.add_argument('-m', dest='vmode', nargs=1, type=str,
metavar='video_mode', choices=['ntsc', 'pal', 'film', 'ivtc'],
default='ntsc', help='Valid values are "ntsc", "pal", "film" and "ivtc".')
...more arguments...
opts=parser.parse_args()

If I assign a value on the command line (e.g. -m pal), opts.vmode is a
list, otherwise it's a string. This is pretty bad since I can't know
whether to get opts.vmode or opts.vmode[0] later in the script. I could
loop through all the options and convert each option to a string, but
that's not really something I want to do, especially if I start adding
more options.

I'm pretty new to Python, and I might have missed something, but I have
been looking!

http://docs.python.org/py3k/library/argparse.html#nargs

"""
Note that nargs=1 produces a list of one item. This is different from the
default, in which the item is produced by itself.
"""

So just omit 'nargs=1' from the add_argument() call.