Argparse optional positional arguments?

Question

I have a script which is meant to be used like this:

usage: installer.py 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.

Answer 1

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:
 dir
optional arguments:
 -h, --help show this help message and exit
 -v

• Dolan Antenucci: Do the ? and * mean the same thing they mean in regular expressions (i.e. ? requires 0 or 1, and * requiring 0 or more)? If so, does + work as well?
• Vinay Sajip: @dolan: Yes, + works, too. See docs.python.org/2/library/argparse.html#nargs for the details.
• scagnetti: is there some way to get dir to show up in optional arguments? or it seems that positional arguments should have a preceeding 'optional' qualifier. is it possible to register (as far as help is concerned) it as such?
• Vinay Sajip: @ant From the above, you can see that dir is optional (that it appears in square brackets in argparse output indicates this).
• Gabriel Staples: Here's the updated (Python 3) documentation--a careful reading of it explains it all: docs.python.org/3/library/argparse.html#nargs. For anyone new to the argparse module, start with the tutorial: docs.python.org/3/howto/argparse.html

Answer 2

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

2. 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.

3. 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.

4. 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.

• Asclepius: It should be noted however that nargs='?' does not produce a list.
• Matas Vaitkevicius: @A-B-B Last line of the answer Generally this means a single command-line argument will be consumed and a single item (not a list) will be produced. Hope this helps...
• Asclepius: The quoted line refers to the case of not defining nargs, but nargs='?' is defining it. 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.
• Matas Vaitkevicius: @A-B-B Just edit the answer if you feel that something is missing. Thanks.
• user8554766: What is the difference between nargs=argparse.REMAINDER and nargs='*', as it seems to me, they are identical in their effect (tested in Python 2.7.10 and Python 3.6.1)?

Answer 3

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:

myfile.py

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 myfile.py
Installing to /users/myname/myfolder
$ python myfile.py /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,

myfile.py

import argparse
parser = argparse.ArgumentParser()
parser.add_argument(
 "-v",
 "--version",
 action="store_true")
parsed_args = parser.parse_args()
if parsed_args.version:
 print("version flag given")
else:
 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 myfile.py -v
version flag given
$ python myfile.py --version
version flag given
$ python myfile.py
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.

myfile.py

import argparse
import pathlib
parser = argparse.ArgumentParser()
parser.add_argument(
 "-i",
 "--installdir", # Optional (but recommended) long version
 type=pathlib.Path,
 default="/bin"
 )
parsed_args = parser.parse_args()
print("Installing to", parsed_args.installdir)

$ python myfile.py -i /usr/bin/
Installing to /usr/bin
$ python myfile.py --installdir /usr/bin/
Installing to /usr/bin
$ python myfile.py
Installing to /bin

Answer 4

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()

• SoloPilot: OP was asking about positional params, not '--dir'. 'required' is an invalid argument for positionals. And 'false' was a typo, she meant 'False'. +1 for newbie, -1 for sloppiness.
• Karthik Sunil: We can not use required for positional argument.