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Using `choices` option in `argparse` module caused unwanted behavior.
# Without `choices`
 parser = argparse.ArgumentParser()
 parser.add_argument('length')
 parser.print_help()
## Output
 usage: demo.py [-h] length
 positional arguments:
 length
 optional arguments:
 -h, --help show this help message and exit
# With choices
 parser = argparse.ArgumentParser()
 parser.add_argument('length', choices=[1,2,3])
 parser.print_help()
## Output
 usage: demo.py [-h] {1,2,3}
 positional arguments:
 {1,2,3}
 optional arguments:
 -h, --help show this help message and exit
I think the doc says ArgumentParser objects use the `dest` value as the "name" of each object. And the doc doesn't mention anything about `dest` be changed by `choices`. The help text looks ugly when there is a long list of choices.
Currently the problem can be remedied by adding a `metavar` option. But I hope it will be fixed in the library itself.