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: 
Use [`nargs='?'`](https://docs.python.org/3/library/argparse.html#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

```
As an extension to @VinaySajip answer. [There are additional `nargs` worth mentioning](https://docs.python.org/2/library/argparse.html#nargs).

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](https://docs.python.org/3/library/argparse.html#nargs) 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.