def truncate(datetime, truncate_to='day'):
    '''
    Truncates a datetime to have the values with higher precision than
    the one set as `truncate_to` as zero (or one for day and month).

    Possible values for `truncate_to`:

    * second
    * minute
    * hour
    * day
    * week (iso week i.e. to monday)
    * month
    * quarter
    * half_year
    * year

    Examples::

       >>> truncate(datetime(2012, 12, 12, 12), 'day')
       datetime(2012, 12, 12)
       >>> truncate(datetime(2012, 12, 14, 12, 15), 'quarter')
       datetime(2012, 10, 1)
       >>> truncate(datetime(2012, 3, 1), 'week')
       datetime(2012, 2, 27)

    :params datetime: an initialized datetime object
    :params truncate_to: The highest precision to keep its original data.
    :return: datetime with `truncated_to` as the highest level of precision
    :rtype: :py:mod:`datetime` datetime object
    '''
    if truncate_to in PERIODS:
        return datetime.replace(**PERIODS[truncate_to])
    elif truncate_to in ODD_PERIODS:
        if truncate_to == 'week':
            return truncate_week(datetime)
        elif truncate_to == 'quarter':
            return truncate_quarter(datetime)
        elif truncate_to == 'half_year':
            return truncate_half_year(datetime)
    else:
        raise ValueError('truncate_to not valid. Valid periods: {}'.format(
            ', '.join(PERIODS.keys() + ODD_PERIODS)
        ))