human
Solve the following python programming problem given the following code:
@directive('variants')
def variant(name, default=None, description='', values=None, multi=False, validator=None):
assistant
#Here is the Python program you requested.

    """Define a variant for the package. Packager can specify a default
    value as well as a text description.
    Args:
    name (str): name of the variant
    default (str or bool): default value for the variant, if not
    specified otherwise the default will be False for a boolean
    variant and \nothing\ for a multi-valued variant
    description (str): description of the purpose of the variant
    values (tuple or callable): either a tuple of strings containing the
    allowed values, or a callable accepting one value and returning
    True if it is valid
    multi (bool): if False only one value per spec is allowed for
    this variant
    validator (callable): optional group validator to enforce additional
    logic. It receives a tuple of values and should raise an instance
    of SpackError if the group doesn\t meet the additional constraints
    """
    if (values is None):
        if ((default in (True, False)) or ((type(default) is str) and (default.upper() in ('TRUE', 'FALSE')))):
            values = (True, False)
        else:
            values = (lambda x: True)
    if (default is None):
        default = (False if (values == (True, False)) else '')
    default = default
    description = str(description).strip()
    def _execute(pkg):
        if (not re.match(spack.spec.identifier_re, name)):
            directive = 'variant'
            msg = "Invalid    variant    name    in    {0}:    '{1}'"
            raise DirectiveError(directive, msg.format(pkg.name, name))
        pkg.variants[name] = Variant(name, default, description, values, multi, validator)
    return _execute