argparse: use of double dash to separate options and positional arguments

[Peter Otten]

Amit Ramon wrote:

> Hello,
> 
> I'm trying to use argparse in a program that gets some options and
> positional arguments. I want to collect all the positional arguments
> in a single argument using the REMAINDER option to add_argument() as
> shown bellow:
> 
>     parser = argparse.ArgumentParser(description='Test argparse')
>     parser.add_argument('-v', '--verbose', action='store_true')
>     parser.add_argument('cmd_args', nargs=argparse.REMAINDER)
>     print parser.parse_args()
> 
> This works well unless the first positional argument starts with a
> '-'.
> 
> For example, with the above code
> 
>     my_prog -v hello world
> 
> works well, but
> 
>     my_prog -v -x hello world
> 
> Fails with an error message 'error: unrecognized arguments: -x'.

This looks like a bug to me. Please report it on bug.python.org.
 
> If I invoke the program with a '--' at the end of the options (as I
> understand is a common standard and a POSIX recommendation), as in:
> 
>     my_prog -v -- -x hello world
> 
> It works well again.
> 
> However, a few questions remain:
> 
> Firstly, as far as I can tell the use of '--' is not documented in the
> argparse documentation (but the code of argparse clearly shows that it
> is a feature).
> 
> Secondly, when using '--', this string itself is inserted into the
> list of the collected positional arguments (in the above example in
> the cmd_args variable):
> 
> $ ./my_prog -v -- -x hello world
> Namespace(cmd_args=['--', '-x', 'hello', 'world'], verbose=True)
> 
> It's quiet easy to check for it and remove it if it exists, but it
> still seems like it shouldn't be there - it is not really an argument,
> just a delimiter.

I'm not sure about this one; one purpose of REMAINDER is to pass on the 
unprocessed arguments to another program/script, and this might follow the 
same convention. Should

parser.add_argument('-v', '--verbose', action='store_true')
parser.add_argument('cmd_args', nargs=argparse.REMAINDER)
args = parser.parse_args()
subprocess.call(["rm"] + args.cmd_args)

$ my_prog -v -- -r foo

attempt to delete two files "-r" and "foo" or remove the "foo" directory? 
The first is the safer alternative, and as you say stripping the "--" is 
easy.


> Am I doing anything wrong, or is this a bug? I hope someone here can
> shed some light on this.