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: 
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.
`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()

```