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

[Peter Otten]

Amit Ramon wrote:

> Peter Otten <__peter__ at web.de> [2015-11-06 11:57 +0100]:
> 
> 
>>>
>>> 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.
> 
> Why does it looks like a bug? To me it seems okay - argparse expects
> that an argument that starts with '-' is an option, and it also expects
> to be told about all possible options, so it complains on
> encountering an unrecognized 'option'. This is why a special delimiter
> is needed to mark the 'end of options'.
> 
> 
>>
>>> 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.
> 
> I'm using the REMAINDER exactly for passing on arguments to another
> program, as in your example. I would expect that everything after
> the '--' should be passed on as is - it should be up to the other
> program to decide what to do with the arguments it receives (So in
> your examples, rm would actually try to remove the "foo" directory).
> 
> And although stripping the '--' is easy, why should the user of
> argparse need to handle it? Still unclear to me.

My interpretation was that REMAINDER should give you the rest of the 
commandline args, no matter what special meaning these args might have for 
argparse.