<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cast_datetime_filter(value):
 """Cast a datetime filter value. :param value: string representation of a value that needs to be casted to a `datetime` object. """

    if isinstance(value, str):
        dtime = parse_datetime(value)

    elif isinstance(value, datetime):
        dtime = value
    else:
        raise ValueError('Received value of type {0}'.format(type(value)))

    return dtime.isoformat()