def is_group(value):
    """
    Check whether groupname or gid as argument exists.
    if this function recieved groupname, convert gid and exec validation.
    """

    if type(value) == str:
        try:
            entry = grp.getgrnam(value)
            value = entry.gr_gid
        except KeyError:
            err_message = ('{0}: No such group.'.format(value))
            raise validate.VdtValueError(err_message)

        return value

    elif type(value) == int:
        try:
            grp.getgrgid(value)
        except KeyError:
            err_message = ('{0}: No such group.'.format(value))
            raise validate.VdtValueError(err_message)

        return value

    else:
        err_message = ('Please, use str or int to "user" parameter.')
        raise validate.VdtTypeError(err_message)
Check whether groupname or gid as argument exists.
    if this function recieved groupname, convert gid and exec validation.