Python: 如何终止无限循环？

Question

我有一个 for 循环进入一个有 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。我想在列表长度为 100 时结束循环，这是可能的还是我无法退出循环，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，但它一直在继续。

有什么想法吗？

Answer 1

您需要在 loop.that 限制循环继续之前检查长度。

Answer 2

如果 len(keywords) 永远不会 == 100.
怎么办因为keywords.append(k)在循环中。
示例：len(keywords) == 98 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 3

这是因为你有 2 个循环，所以你必须中断两次，我不认为这是最优化的解决方案，但它可以完成工作：

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 4

您应该避免修改正在循环访问的列表，因为这会导致奇怪的行为。最好有一个尚未尝试过的查询列表，你可以边走边用尽和填充，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：

可能是你在做什么

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。外层一直在继续，所以你有一个无限循环，因为它不断撞击内层循环然后一次又一次地爆发，因为外层循环永远不会终止。

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

到这里，外循环就结束了。但是，它可能不会，因为中断条件的表述很糟糕。由于您可以在每个内部循环中添加多个单词，因此 len(keywords) 可能会从 <100 变为 >100 而不会达到条件。然后它永远不会，你又会得到一个无限循环。相反，它应该是 len(keywords) >= 100，尽管这样你会遇到 keywords 不完全是 100 的情况。

但真正的答案是你不应该首先修改你正在迭代的列表，并相应地重构你的逻辑。

Answer 5

通常在遍历列表时修改列表被认为是不好的做法。话虽这么说，但这是一种方法：

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])

Python: 如何终止无限循环？

Python: How to kill a infinite for loop?

python

logic

for-loop

版本 1：

版本 2：