没有明显原因的字典更改

Question

出于学习原因，我正在尝试创建一个琐事游戏，但是我的字典发生了一些我无法理解的奇怪问题。

我通常测试的代码是这样的：

cat = {
    'easy': {
        'books': 'https://opentdb.com/api.php?amount=50&category=10&difficulty=easy&type=boolean',
        'films': 'https://opentdb.com/api.php?amount=50&category=11&difficulty=easy&type=boolean',
        'general knowledge': 'https://opentdb.com/api.php?amount=50&category=9&difficulty=easy&type=boolean',
        'music': 'https://opentdb.com/api.php?amount=50&category=12&difficulty=easy&type=boolean',
        'science & nature': 'https://opentdb.com/api.php?amount=50&category=17&difficulty=easy&type=boolean',
        'sports': 'https://opentdb.com/api.php?amount=50&category=21&difficulty=easy&type=boolean'
    },
    'medium': {
        'books': 'https://opentdb.com/api.php?amount=50&category=10&difficulty=medium&type=boolean',
        'films': 'https://opentdb.com/api.php?amount=50&category=11&difficulty=medium&type=boolean',
        'general knowledge': 'https://opentdb.com/api.php?amount=50&category=9&difficulty=medium&type=boolean',
        'music': 'https://opentdb.com/api.php?amount=50&category=12&difficulty=medium&type=boolean',
        'science & nature': 'https://opentdb.com/api.php?amount=50&category=17&difficulty=medium&type=boolean',
        'sports': 'https://opentdb.com/api.php?amount=50&category=21&difficulty=medium&type=boolean'
    }
}

print(cat)

for level in cat:
    print(level)

catselect = []
while catselect not in ("1", "2"):
    catselect = input("Select a category, for easy press 1, for medium press 2: ")

    if catselect == "1":
        selectedcat = "easy"
    elif catselect == "2":
        selectedcat = "medium"
    print(f"You selected the {selectedcat} difficulty level")

    print("The subjects can be: ")
    for i, cat[selectedcat] in enumerate(cat[selectedcat]):
        print(i, cat[selectedcat])

print(cat)

所以当最后代码运行时，猫字典是不一样的 更多，我没有任何理由会发生这种情况。

这是我看到的：

{'easy': {'books': 'https://opentdb.com/api.php?amount=50&category=10&difficulty=easy&type=boolean', 'films': 'https://opentdb.com/api.php?amount=50&category=11&difficulty=easy&type=boolean', 'general knowledge': 'https://opentdb.com/api.php?amount=50&category=9&difficulty=easy&type=boolean', 'music': 'https://opentdb.com/api.php?amount=50&category=12&difficulty=easy&type=boolean', 'science & nature': 'https://opentdb.com/api.php?amount=50&category=17&difficulty=easy&type=boolean', 'sports': 'https://opentdb.com/api.php?amount=50&category=21&difficulty=easy&type=boolean'}, 'medium': {'books': 'https://opentdb.com/api.php?amount=50&category=10&difficulty=medium&type=boolean', 'films': 'https://opentdb.com/api.php?amount=50&category=11&difficulty=medium&type=boolean', 'general knowledge': 'https://opentdb.com/api.php?amount=50&category=9&difficulty=medium&type=boolean', 'music': 'https://opentdb.com/api.php?amount=50&category=12&difficulty=medium&type=boolean', 'science & nature': 'https://opentdb.com/api.php?amount=50&category=17&difficulty=medium&type=boolean', 'sports': 'https://opentdb.com/api.php?amount=50&category=21&difficulty=medium&type=boolean'}}
easy
medium
Select a category, for easy press 1, for medium press 2: 1
You selected the easy difficulty level
The subjects can be: 
0 general knowledge
1 books
2 films
3 music
4 sports
5 science & nature
{'easy': 'science & nature', 'medium': {'general knowledge': 'https://opentdb.com/api.php?amount=50&category=9&difficulty=medium&type=boolean', 'books': 'https://opentdb.com/api.php?amount=50&category=10&difficulty=medium&type=boolean', 'films': 'https://opentdb.com/api.php?amount=50&category=11&difficulty=medium&type=boolean', 'music': 'https://opentdb.com/api.php?amount=50&category=12&difficulty=medium&type=boolean', 'sports': 'https://opentdb.com/api.php?amount=50&category=21&difficulty=medium&type=boolean', 'science & nature': 'https://opentdb.com/api.php?amount=50&category=17&difficulty=medium&type=boolean'}}

easy 的所有类别都去了哪里？为什么我最终会选择'science & nature'？

Answer 1

您在此处分配回字典：

for i, cat[selectedcat] in enumerate(cat[selectedcat]):

您要求for 循环分配给i 和cat[selectedcat]。不要那样做。

上面基本上是这样做的：

iterator = iter(enumerate(cat[selectedcat]))
while True:
    try:
        next_item = next(iterator)
    except StopIteration:
        break
    i, cat[selectedcat] = next_item

因此将每个值从cat[selectedcat] 分配给cat[selectedcat] 本身。

它恰好起作用，因为 cat[selectedcat] 引用的原始字典仍然被 enumerate() 对象引用，因此它的所有键仍在循环中生成。但是cat 字典本身被要求用每个类别字符串依次替换selectedcat 键的值。您可以看到这种情况发生，因为您随后在循环内 打印 cat[selectedcat] 的新值。

如果你想用数字显示值，你想为循环使用不同的新变量名，例如：

for i, category in enumerate(cat[selectedcat]):
    print(i, category)

这里，category 是一个新变量（就像 i 一样）。

Answer 2

问题出在这一行：

    for i, cat[selectedcat] in enumerate(cat[selectedcat]):

每次循环都会将cat[selectedcat] 的元素分配给cat[selectedcat]，从而修改字典。所以第一次通过循环，它确实

cat[selectedcat] = cat[selectedcat][0]

当循环完成时，cat[selectedcat] 的值将是cat[selectedcat] 的最后一个元素。

您通常应该为循环变量使用普通变量：

for i, value in enumerate(cat[selectedcat]):
    print(i, value)