Final script for https://www.youtube.com/watch?v=GRbOVcJWFq8

packaging.py

import copy
import math


def box(remaining, capacity):
    if len(remaining) >= capacity:
        weights = [b.item_weight for b in remaining[:capacity]]
        return sum(weights)


def package(bottles):
    remaining = bottles
    packages = []

    num_12 = math.floor(len(remaining) / 12)
    for _ in range(0, num_12):
        weights = [b.item_weight for b in remaining[:12]]
        packages.append(sum(weights))
        remaining = remaining[12:]

    for capacity in (9, 6, 3):
        box_weight = box(remaining, capacity)
        if box_weight:
            packages.append(box_weight)
            remaining = remaining[capacity:]

    for bottle in remaining:
        packages.append(bottle.item_weight)

    return packages

test_packaging.py

import pytest
import random
from shipping.packaging import package


class BottleMock:
    def __init__(self, weight=1):
        self.item_weight = weight


def scenario(*args):
    return [1 for _ in range(0, sum(args))], list(args)


def test_scenario():
    assert scenario(3, 1) == ([1, 1, 1, 1], [3, 1])


@pytest.mark.parametrize('weigths,packages', (
    # test that we sum the weight properly
    ([1, 3, 2, 2], [6, 2]),
    # now test that we package properly :)
    scenario(3, 1),
    scenario(6, 1, 1),
    scenario(9, 1),
    scenario(12, 12, 6, 1),
))
def test_package(weigths, packages):
    bottles = [BottleMock(w) for w in weigths]
    assert package(bottles) == packages