Python's argparse choices constrained printing

Question

Currently I want Python's argparse module to only print out '1 - 65535' rather than {1, 2, 3, ... 65535}, but the documentation doesn't seem to provide any method for this. Any suggestions?

Answer 1

You can alter the way defaults are formatted by setting the formatter_class option.

I'd subclass the HelpFormatter class to alter the way it formats your choices values. This class is officially an "implementation detail" but I doubt it'll change much with newer python versions.

The _metavar_formatter method formats the {1, 2, ..., 65535} string and your subclass could override that:

class RangeChoiceHelpFormatter(HelpFormatter):
    def _metavar_formatter(self, action, default_metavar):
         if action.metavar is not None:
             result = action.metavar
         elif action.choices is not None:
             result = '{%s .. %s}' % (min(action.choices), max(action.choices])
         else:
             result = default_metavar

          def format(tuple_size):
              if isinstance(result, tuple):
                  return result
              else:
                  return (result, ) * tuple_size
          return format

Another option is to not use the choices argument for such a large range, and instead define a new argument type.

This is just a callable, passed a string, that raises argparse.ArgumentTypeError, TypeError or ValueError if the string cannot be converted to the target type, or the converted value otherwise:

class IntRange(object):
    def __init__(self, start, stop=None):
        if stop is None:
            start, stop = 0, start
        self.start, self.stop = start, stop

    def __call__(self, value):
        value = int(value)
        if value < self.start or value >= self.stop:
            raise argparse.ArgumentTypeError('value outside of range')
        return value

You can use this as the argument type like this:

parser.add_argument('foo', type=IntRange(1, 65536))

and adjust your help message to indicate what values are acceptable.