有人可以解释一下这个错误吗 - 运行时错误：迭代期间字典大小发生了更改？

Question

def find_the_best_solution(shipping_request, list, current_solution, best_shipping_solution):
    if shipping_request == {}:
        return current_solution

    for item in shipping_request:
        for i in range(0,len(list), 1):
            length_of_combination = i+1
            combinations = itertools.combinations(list, length_of_combination)
            for x in combinations:
                if sum(x) >= shipping_request[item]:
                    if sum(x) > shipping_request[item]:
                        current_solution['wastage'] += (sum(x) - shipping_request[item])
                    current_solution['number_of_containers'] += length_of_combination
                    current_solution['distribution'][item] = x
                    temp_request = shipping_request
                    del temp_request[item]
                    temp_list = list
                    for y in x:
                        temp_list.remove(y)
                    solution = find_the_best_solution(temp_request, temp_list, current_solution, best_shipping_solution)
                    if solution['wastage'] < best_shipping_solution['wastage']:
                        best_shipping_solution = solution
                    elif solution['wastage'] == best_shipping_solution['wastage']:
                        if solution['number_of_containers'] < best_shipping_solution['number_of_containers']:
                            best_shipping_solution = solution

    return best_shipping_solution   

这将产生以下错误-“运行时错误：迭代期间字典大小更改”

我做错什么了？我该怎么做呢？在

Answer 1

这与itertools.combinations无关，您试图更改正在迭代的数据的结构，这是不允许的：

Runtime error: dictionary size changed during iteration

您试图在迭代字典时添加或删除字典中的键。一种解决方法是迭代for key in list(dictionary)，它生成字典键的副本列表，因此您仍然可以修改底层字典，但是您可能还需要重新考虑您的一般方法。在

导致错误的行包括：

^{pr2}$

也许你想做一个浅拷贝，比如temp_request = dict(shipping_request)，它生成一个新字典，其中引用了相同的对象。但是，您可能希望import copy; copy.deepcopy递归地复制字典中的每个对象。在