class FourDigitYearConverter:
    regex = "[0-9]{4}"

    def to_python(self, value):
        return int(value)

    def to_url(self, value):
        return str(value).zfill(4)


class TwoDigitMonthConverter:
    regex = "[0-9]{2}"

    def to_python(self, value):
        return int(value)

    def to_url(self, value):
        return str(value).zfill(2)


class BooleanStringConverter:
    """Converter whose regex match either True or False."""

    regex = r"(True)|(False)"

    def to_python(self, value):
        return str(value) == "True"

    def to_url(self, value):
        return str(value)
