Format an arrow datetime object.

    :param value: The arrow datetime object.
    :param format_string: The date format string
    :returns: Returns a string representation of the given arrow datetime
        object, formatted according to the given format string.

    .. note::
        Do not use this filter to format date/times presented to an end
        user. Instead use ``datetimeformat`` or ``dateformat`` from
        Invenio-I18N.

def format_arrow(value, format_string):
    """Format an arrow datetime object.

    :param value: The arrow datetime object.
    :param format_string: The date format string
    :returns: Returns a string representation of the given arrow datetime
        object, formatted according to the given format string.

    .. note::
        Do not use this filter to format date/times presented to an end
        user. Instead use ``datetimeformat`` or ``dateformat`` from
        Invenio-I18N.
    """
    assert isinstance(value, arrow.Arrow)
    return value.format(format_string)