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.
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**

```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())

```

```bash
$ 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.

```bash
$ 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**

```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)

```

```bash
$ 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

```