def on_participant_deletion()

in api/choice_algorithm.py [0:0]

• 19 lines of code
• 6 McCabe index (conditional complexity)

def on_participant_deletion(wheel, participant):
    """
    Normalize the remaining participant weights to account for participant removal.
    The ratio is based on the following:
     1) The participant should be at weight=1 when it leaves the system (which is the same as it arrived)
     2) That difference should be split by the remaining participants proportional by weight
        This ensures that 'weight=0' participants are still at weight=0 and that the sum of all
        weights is equal to the number of participants, so new additions are treated fairly
    :param wheel: Wheel dictionary:
    {
      "id": string ID of the wheel (DDB Hash Key),
      "name": string name of the wheel,
      "participant_count": number of participants in the wheel,
    }
    :param participant: Participant dictionary:
    {
      "id": string ID of the wheel (DDB Hash Key),
      "name": string name of the wheel,
      "url": Participant's URL,
      "wheel_id": string ID of the wheel the participant belongs to,
    }
    :return: None
    """
    total_weight = participant['weight']
    for p in WheelParticipant.iter_query(KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
        total_weight += p['weight']

    weight = participant['weight']
    remaining_weight = total_weight - weight  # <-- no longer presumes existing weight balance via 'int(participant_count)'
    ratio = (1 + ((weight - 1) / remaining_weight)) if (remaining_weight != 0) else 1
    num_participants = Decimal(0)
    with WheelParticipant.batch_writer() as batch:
        for p in WheelParticipant.iter_query(KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
            if p['id'] != participant['id']:
                # This is cast to a string before turning into a decimal because of rounding/inexact guards in boto3
                p['weight'] = Decimal(str(float(p['weight']) * float(ratio))) if (remaining_weight != 0) else \
                    get_sub_wheel_size(p['name'])
                batch.put_item(Item=p)
                num_participants = num_participants+1

    Wheel.update_item(
        Key={'id': wheel['id']},
        **to_update_kwargs({'participant_count': num_participants})
    )