Python argparse treat arguments in different ways

Question

I want to create an optional argument, which will be the '-- ' (double dash and a space) and get everything after it as its value. The problem is that some other optional arguments might appear after '-- '. I don't want these to be parsed as optional arguments, but as the values of '-- '. For example:

python prog1 --foo 1 --bar 2

Here foo and bar are optional arguments with values 1 and 2 respectively

python prog1 --foo 1 --bar 2 -- --foo 4 --bar 14

Here I want foo and bar that come before '-- ' to be parsed as optional arguments. But I want '--foo 4 --bar 14' to be parsed as the value of the optional argument '-- '. I would like to do this without renaming the parameters that come after '-- ' to foo2 and bar2, if possible.

So is this possible? And how could this be implemented?

Why do you want to do this? It's a complete antipattern, and will likely just confuse people (/ your future self). — will
– will, Commented Sep 16, 2014 at 15:19
@will -- This isn't a complete anti-pattern. This is actually a relatively common way to specify arguments that will be passed to a slave program (maybe via subprocess.Popen). — mgilson
– mgilson, Commented Sep 16, 2014 at 15:21
In fact, argparse already supports the common convention of using -- to signal the end of option parsing. — chepner
– chepner, Commented Sep 16, 2014 at 15:41
-- is commonly used on Unix-like OSes to separate arguments from raw input (often the raw input represents the arguments to be passed to some other command, as in the case of xargs). See unix.stackexchange.com/q/11376/4322. — Vicky Chijwani
– Vicky Chijwani, Commented Feb 10, 2017 at 13:29

chepner · Accepted Answer · 2019-06-06 15:38:44Z

17

argparse already supports -- as an end-of-options terminator. The only difference is that the default handling treats the following arguments individually. Suppose prog1 simply looks like

import argparse

p = argparse.ArgumentParser()

p.add_argument('--foo')
p.add_argument('--bar')
p.add_argument('remaining', nargs=argparse.REMAINDER)

print p.parse_args()

Then the command

% python prog1 --foo 1 --bar 2 -- --foo 4 --bar 14

produces

Namespace(bar='2', foo='1', remaining=['--', '--foo', '4', '--bar', '14'])

(rather than Namespace(..., remaining='-- --foo 4 --bar 14')). Than can be fixed with a quick post-processing step, if necessary:

args = p.parse_args()
args.remaining = " ".join(args.remaining)
print args

Then the result is

Namespace(bar='2', foo='1', remaining='-- --foo 4 --bar 14')

Discarding the -- argument requires some additional post-processing:

args = p.parse_args()
if args.remaining[0] == "--":
    args = args[1:]

edited Jun 6, 2019 at 15:38

answered Sep 16, 2014 at 15:40

chepner

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6 Comments

mgilson Over a year ago

This is brilliant -- however, I can't seem to find it in the docs. The closest that I have been able to find is argparse.REMAINDER, but that doesn't seem to use the -- separator...

chepner Over a year ago

See the very end of section 15.4.4.3 (link to the Python 2.7 version of the documentation).

chepner Over a year ago

Thanks for reminding me about REMAINDER; that seems like a better choice than * (at least more descriptive; I can't tell if there is a functional difference in this case.)

mgilson Over a year ago

Nice. Well, you have my +1 anyway -- and I learned something new.

gsgx Over a year ago

I ran this code and my remaining was remaining=['--', '--foo', '4', '--bar', '14']. Note the -- showed up in the remaining.

|

will · Accepted Answer · 2014-09-16 15:30:28Z

1

It looks like the docopt library can do this for you;

From the usage-pattern-format section:

"[--]". Double dash "--" is used by convention to separate positional arguments that can be mistaken for options. In order to support this convention add "[--]" to your usage patterns.

answered Sep 16, 2014 at 15:30

will

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