def coerce(cls, string):
        """Create a :class:`Requirement` object from a given package spec."""
        match = re.match(cls.spec_regex, string)
        if not match:
            raise InvalidRequirementSpecError("could not parse requirement")
        package = match.group(1)
        if all(match.group(2, 3)):
            version = "".join(match.group(2, 3))
        else:
            version = None
        return cls(package, version)
def test_invalid_isointerval_error():
    try:
        inputs.iso8601interval('2013-01-01/blah')
    except ValueError as error:
        assert_equal(
            str(error),
            "Invalid argument: 2013-01-01/blah. argument must be a valid ISO8601 "
            "date/time interval.",
        )
        return
    assert False, 'Should raise a ValueError'