optparse 'constructor' option demo

optparse_demo.py

from copy import copy
from optparse import Option, OptionParser, OptionValueError


class Stars(object):
    def __init__(self, stars):
        stars = float(stars)
        if stars < 0.5 or stars > 5 or stars % 0.5 != 0:
            raise ValueError(
                "Stars must be a whole or half number between 0.5 and 5")
        self.stars = stars

    def __str__(self):
        return "{} stars!".format(self.stars)


def check_constructor(option, opt, value):
    con = option.constructor
    assert con is not None, "Oops! Developer forgot to set 'constructor' kwarg"
    try:
        return con(value)
    except Exception as e:
        raise OptionValueError("option {} invalid: {}".format(opt, e))


class MyOption(Option):
    ATTRS = Option.ATTRS + ["constructor"]
    TYPES = Option.TYPES + ("constructor",)
    TYPE_CHECKER = copy(Option.TYPE_CHECKER)
    TYPE_CHECKER["constructor"] = check_constructor


parser = OptionParser(option_class=MyOption)
opt = parser.add_option(
    "--rating",
    type='constructor',
    constructor=Stars,
)

print parser.parse_args(['--rating=3'])[0].rating
print parser.parse_args(['--rating=0.5'])[0].rating
print parser.parse_args(['--rating=0.6'])[0].rating