Argparse optional positional arguments?


I have a script which is meant to be used like this:
usage: dir [-h] [-v]

dir is a positional argument which is defined like this:

parser.add_argument('dir', default=os.getcwd())

I want the dir to be optional: when it’s not specified it should just be cwd.

Unfortunately when I don’t specify the dir argument, I get Error: Too few arguments.

Asked By: Waldo Bronchart



Use nargs='?' (or nargs='*' if you need more than one dir)

parser.add_argument('dir', nargs='?', default=os.getcwd())

extended example:

>>> import os, argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('-v', action='store_true')
_StoreTrueAction(option_strings=['-v'], dest='v', nargs=0, const=True, default=False, type=None, choices=None, help=None, metavar=None)
>>> parser.add_argument('dir', nargs='?', default=os.getcwd())
_StoreAction(option_strings=[], dest='dir', nargs='?', const=None, default='/home/vinay', type=None, choices=None, help=None, metavar=None)
>>> parser.parse_args('somedir -v'.split())
Namespace(dir='somedir', v=True)
>>> parser.parse_args('-v'.split())
Namespace(dir='/home/vinay', v=True)
>>> parser.parse_args(''.split())
Namespace(dir='/home/vinay', v=False)
>>> parser.parse_args(['somedir'])
Namespace(dir='somedir', v=False)
>>> parser.parse_args('somedir -h -v'.split())
usage: [-h] [-v] [dir]

positional arguments:

optional arguments:
  -h, --help  show this help message and exit
Answered By: Vinay Sajip

As an extension to @VinaySajip answer. There are additional nargs worth mentioning.

  1. parser.add_argument('dir', nargs=1, default=os.getcwd())

N (an integer). N arguments from the command line will be gathered together into a list

  1. parser.add_argument('dir', nargs='*', default=os.getcwd())

‘*’. All command-line arguments present are gathered into a list. Note that it generally doesn’t make much sense to have more than one positional argument with nargs='*', but multiple optional arguments with nargs='*' is possible.

  1. parser.add_argument('dir', nargs='+', default=os.getcwd())

‘+’. Just like ‘*’, all command-line args present are gathered into a list. Additionally, an error message will be generated if there wasn’t at least one command-line argument present.

  1. parser.add_argument('dir', nargs=argparse.REMAINDER, default=os.getcwd())

argparse.REMAINDER. All the remaining command-line arguments are gathered into a list. This is commonly useful for command line utilities that dispatch to other command line utilities

If the nargs keyword argument is not provided, the number of arguments consumed is determined by the action. Generally this means a single command-line argument will be consumed and a single item (not a list) will be produced.

Edit (copied from a comment by @Acumenus) nargs='?' The docs say: ‘?’. One argument will be consumed from the command line if possible and produced as a single item. If no command-line argument is present, the value from default will be produced.

Answered By: Matas Vaitkevicius

parser.add_argument also has a switch required. You can use required=False.
Here is a sample snippet with Python 2.7:

parser = argparse.ArgumentParser(description='get dir')
parser.add_argument('--dir', type=str, help='dir', default=os.getcwd(), required=False)
args = parser.parse_args()
Answered By: rakhee

Short Answer

As already shown in the previous two answers, you can accept an optional positional argument with nargs='?'. You could also turn the argument directly into a Path type and/or shorten the cwd to . if you wanted to:

import argparse
import pathlib

parser = argparse.ArgumentParser()
parser.add_argument("dir", nargs="?", default=".", type=pathlib.Path)
parsed_args = parser.parse_args()

print("Installing to", parsed_args.dir.resolve())
$ python
Installing to /users/myname/myfolder

$ python /usr/bin/
Installing to /usr/bin

Longer answer

Since you also mention the flag-style True/False options -h and -v in your question, these examples may be of use:

Flags (e.g. -v)

We might refer to optional options that take no arguments as "flags". With flags, we only care about whether they are given or not. -h is a flag that argparse adds automatically (along with the longer version --help) so we shouldn’t really override that. If we consider -v then,

import argparse

parser = argparse.ArgumentParser()
parsed_args = parser.parse_args()

if parsed_args.version:
    print("version flag given")
    print("version flag not given")

Note that the second argument to add_argument() is a longer name for the option. It is not mandatory but it does make your subsequent code more readable (parsed_args.version vs parsed_args.v) and makes calls to your installer more explicit.

$ python -v
version flag given

$ python --version
version flag given

$ python
version flag not given

Optional arguments (e.g. --installdir /usr/bin/)

One could argue that, in your case, you would be better off with an optional argument rather than a positional one.

import argparse
import pathlib

parser = argparse.ArgumentParser()
        "--installdir",  # Optional (but recommended) long version
parsed_args = parser.parse_args()

print("Installing to", parsed_args.installdir)
$ python -i /usr/bin/
Installing to /usr/bin

$ python --installdir /usr/bin/
Installing to /usr/bin

$ python
Installing to /bin
Answered By: FiddleStix
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