Python：如何杀死无限循环？

Question

我有一个 for 循环进入一个包含 10 个项目的列表。对于它拥有的每个项目，它都会使用 for 循环向列表中添加 10 个。一旦列表长度为 100，我想终止循环，这可能吗？或者我无法退出循环，因为列表永远不会结束增长？

这是我的代码：

keywords = [
"apple store",
"applesauce recipe",
"applesauce",
"appleseeds",
"apples to apples",
"apples and honey",
"applesauce cake",
"apple support",
"apples and bananas"
]

   for i in keywords:
            url = "http://suggestqueries.google.com/complete/search?output=firefox&q=" + i
            response = requests.get(url, verify=False)
            suggestions = json.loads(response.text)
            for word in suggestions[1]:
                k = word
                keywords.append(k)
                print(word)

我尝试在最后添加以下内容，但没有成功

if len(keywords) == 100:
    print('this is where it needs to break')
    break

当它达到 100 时，我看到了打印消息，但它一直在运行。

有什么想法吗？

Answer 1

如果len(keywords) 永远不会是== 100。
因为keywords.append(k) 处于循环中。
示例：len(keywords) == 98 和循环后 len(keywords) 变为 103。

尝试更改if 语句以测试len(keywords) 是否大于或等于100。

for i in keywords:
    url = "http://suggestqueries.google.com/complete/search?output=firefox&q=" + i
    response = requests.get(url, verify=False)
    suggestions = json.loads(response.text)
    for word in suggestions[1]:
        keywords.append(word)
        print(word)

    if len(keywords) >= 100:  # test if greater or equal than 100
        print('this is where it needs to break')
        break

Answer 2

这是因为你有 2 个循环，所以你必须打破两次，我不认为这是最优化的解决方案，但它可以工作：

import requests
import json

keywords = [
"apple store",
"applesauce recipe",
"applesauce",
"appleseeds",
"apples to apples",
"apples and honey",
"applesauce cake",
"apple support",
"apples and bananas"
]

for i in keywords:
    url = "http://suggestqueries.google.com/complete/search?output=firefox&q=" + i
    response = requests.get(url, verify=False)
    suggestions = json.loads(response.text)
    if len(keywords) == 100:
        print('this is where it needs to break')
        break
    else:
        for word in suggestions[1]:
            if len(keywords) == 100:
                print('this is where it needs to break')
                break
            else:
                keywords.append(word)
                print(word)

Answer 3

您应该避免修改您正在迭代的列表，因为它会导致奇怪的行为。最好有一个尚未尝试过的查询列表，您可以随时将其耗尽并填充，以及一个单独的循环（for i in range(100) 或带有中断条件的 while 循环）。

至于您显示的代码，有两个选项，具体取决于您放置中断条件的位置。

版本 1：

可能你在做什么

for i in keywords:
    url = "http://suggestqueries.google.com/complete/search?output=firefox&q=" + i
    response = requests.get(url, verify=False)
    suggestions = json.loads(response.text)
    for word in suggestions[1]:
        k = word
        keywords.append(k)
        print(word)

        if len(keywords) == 100:
            print('this is where it needs to break')
            break

在这里，你只打破了内部循环。外循环一直在运行，所以你有一个无限循环，因为它不断撞击内循环，然后一次又一次地中断，因为外循环永远不会终止。

版本 2：

这仍然很容易产生无限循环。

for i in keywords:
    url = "http://suggestqueries.google.com/complete/search?output=firefox&q=" + i
    response = requests.get(url, verify=False)
    suggestions = json.loads(response.text)
    for word in suggestions[1]:
        k = word
        keywords.append(k)
        print(word)

    if len(keywords) == 100:
        print('this is where it needs to break')
        break

到此，外循环将结束。但是，它可能不会，因为中断条件的制定很糟糕。由于您可以在每个内部循环中添加多个单词，len(keywords) 可能会从<100 变为>100 而不会满足条件。然后它永远不会，你又得到一个无限循环。 相反，它应该是len(keywords) >= 100，不过你会遇到keywords 不完全是100 的情况。

但真正的答案是，您不应该首先修改您正在迭代的列表，并相应地重组您的逻辑。

Answer 4

通常，在遍历列表时修改列表被认为是不好的做法。话虽如此，这是一种方法：

keywords = [
"apple store",
"applesauce recipe",
"applesauce",
"appleseeds",
"apples to apples",
"apples and honey",
"applesauce cake",
"apple support",
"apples and bananas"
]

for i in keywords:
    url = "http://suggestqueries.google.com/complete/search?output=firefox&q=" + i
    response = requests.get(url, verify=False)
    suggestions = json.loads(response.text)
    if len(keywords) >= 100:
        print('this is where it needs to break')
        break
    for word in suggestions[1]:
        k = word
        keywords.append(k) if len(keywords) < 100 else 0 # you can put basically anything here insetad of 0
        print(word)

更好的方法是使用第二个列表并使用 extend() 函数

keywords = [
"apple store",
"applesauce recipe",
"applesauce",
"appleseeds",
"apples to apples",
"apples and honey",
"applesauce cake",
"apple support",
"apples and bananas"
]

new_keywords = list(keywords) # storing a copy of keywords in new_keywords

for i in keywords:
    url = "http://suggestqueries.google.com/complete/search?output=firefox&q=" + i
    response = requests.get(url, verify=False)
    suggestions = json.loads(response.text)
    
    new_keywords.extend(suggestions[1])

Answer 5

您需要检查循环之前的长度。这将限制循环继续进行。