Add angle brackets for required args in argparse

[scruel tao]

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
```

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

Thanks.

[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.

[scruel tao]

Thank you for your workarounds, Mark Bourne.
`metavar` argument should be sufficient for infrequent use scenarios, and I will consider to use the custom help formatter if necessary.

>>> 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).

Your are right to be concerned, that’s why I still think, might the `argparse` can provide a more stable way which can set such format strategy globally, your workarounds are fine to work now, just I have to write the same code to parse either `metaver` or `formatter` every times I use argparse.
Might can have different argparse subclasses, or make some HelpFormatter builtin, so that users won’t need to write them by themselves.