Computes the differences between the existing and the expected SNMP config.

def _compute_diff(existing, expected):

    '''
    Computes the differences between the existing and the expected SNMP config.
    '''

    diff = {}

    for key in ['location', 'contact', 'chassis_id']:
        if existing.get(key) != expected.get(key):
            _create_diff(diff,
                         _valid_str,
                         key,
                         existing.get(key),
                         expected.get(key))

    for key in ['community']:  # for the moment only onen
        if existing.get(key) != expected.get(key):
            _create_diff(diff,
                         _valid_dict,
                         key,
                         existing.get(key),
                         expected.get(key))

    return diff