Add angle brackets for required args in argparse

[Mark Bourne]

scruel tao wrote:
> If we have the following code:
> ```
> parser = argparse.ArgumentParser(description="test")
> parser.add_argument('path')
> ```
> 
> Run it without args, will get error message:
> ```
> usage: test.py [-h] path
> test.py: error: the following arguments are required: path
> ```
> 
> However, I hope the message can be as the following:
> ```
> usage: test.py [-h] <path>
> test.py: error: the following arguments are required: path
> ```

The `metavar` argument to `add_argument` can be used to control how an 
argument is represented in the usage text:
```
import argparse
parser = argparse.ArgumentParser(description='test')
parser.add_argument('path', metavar='<path>')
parser.parse_args()
```

Which results in:
```
usage: test.py [-h] <path>
test.py: error: the following arguments are required: <path>
```

> Or might can consider to provide a way to let user have there own style, like:
> ```
> usage: test.py [-h] path
> ```

It's also possible to create a custom help formatter, overriding 
appropriate methods to control the formatting.  For example:
```
import argparse

class CustomHelpFormatter(argparse.HelpFormatter):
     def _get_default_metavar_for_positional(self, action):
         default = super()._get_default_metavar_for_positional(action)
         return f'<{default}>'

parser = argparse.ArgumentParser(
     description='test',
     formatter_class=CustomHelpFormatter)
parser.add_argument('path')
parser.parse_args()
```

Which results in:
```
usage: test.py [-h] <path>
test.py: error: the following arguments are required: path
```

That's a bit closer to what you asked for, since the required argument 
shown in the error message doesn't include the angle brackets.  It also 
avoids needing to specify a `metavar` for every positional argument. 
However, it is overriding a non-public method of the `HelpFormatter` 
class, so might not work across all Python versions if the name or 
signature of that method changes (even if it does work with all current 
versions, it might break in future).

-- 
Mark.