Web Scraping & Bots · 课时

从 JavaScript 中提取数据

学习获取由 JavaScript 生成或加载的数据，包括 AJAX 调用和单页应用中的内容。

从 JavaScript 中提取数据 是 CoddyKit 上的免费 Web Scraping & Bots 课时。这是第 3 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Web Scraping & Bots 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Web Scraping & Bots 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Dynamic Web Content Explained

Many modern websites don't load all their content at once. Instead, they use JavaScript to fetch data and update the page after it initially loads. This is called dynamic content.

Traditional scraping tools like Requests and BeautifulSoup only see the initial HTML. They miss anything JavaScript loads later.

JavaScript's Role in Loading

JavaScript can load new data in several ways:

AJAX Calls: Asynchronous JavaScript and XML. The browser requests data from a server in the background without reloading the entire page.
DOM Manipulation: JavaScript directly adds, removes, or modifies elements in the page's structure (Document Object Model).
Single-Page Applications (SPAs): Entire websites built to dynamically load content and navigate without full page refreshes.

This dynamic loading makes scraping more complex.

When Requests & BS4 Fall Short

When you use Python's requests library, you get the raw HTML that the server sends initially. If a website then uses JavaScript to load more data, that data won't be in the HTML you received.

BeautifulSoup can only parse the HTML it's given. It can't execute JavaScript to fetch additional content or wait for it to appear.

Selenium for Dynamic Content

This is where Selenium becomes essential. Selenium controls a real web browser (like Chrome or Firefox) programmatically.

When Selenium opens a page, the browser executes all JavaScript, including any AJAX calls or DOM manipulations. This means Selenium sees the fully rendered page, just like a human user would.

Waiting for Elements to Appear

Since content loads dynamically, it might not be immediately available when Selenium first loads a page. You need to tell Selenium to wait until a specific element or condition is met before trying to extract data.

If you don't wait, your script might try to find an element before JavaScript has rendered it, leading to errors or missing data.

Using Explicit Waits

Selenium's WebDriverWait combined with ExpectedConditions (EC) allows you to set explicit waits. This tells Selenium to wait for a maximum amount of time until a certain condition is true.

Common conditions include waiting for an element to be visible, clickable, or present in the DOM.

Try running this example:

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
import time

def main():
    driver = webdriver.Chrome()
    driver.get("https://www.selenium.dev/selenium/web/dynamic.html")

    # Click a button that will make an element appear after a delay
    driver.find_element(By.ID, "adder").click()

    # Wait up to 10 seconds for the new element to appear
    try:
        new_element = WebDriverWait(driver, 10).until(
            EC.presence_of_element_located((By.ID, "box0"))
        )
        print("New element found:", new_element.text)
    except Exception as e:
        print("Element not found within time limit:", e)

    driver.quit()

if __name__ == "__main__":
    main()

Extracting Rendered Data

Once you've successfully waited for dynamic content to appear, extracting the data is similar to how you'd extract from static content using Selenium.

You can use methods like find_element(By.ID, "id_name"), find_element(By.CLASS_NAME, "class_name"), or find_element(By.CSS_SELECTOR, "css_selector") to locate the elements. Then, you can retrieve their text content or attributes.

Code: Get Dynamic Text

This example shows how to wait for an element and then extract its text. Imagine a page where a "Loading..." message changes to actual data after a few seconds.

Try running this example:

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC
import time

def main():
    driver = webdriver.Chrome()
    # Using a simple test page that changes content
    driver.get("https://www.selenium.dev/selenium/web/dynamic.html")

    # Click button to reveal content
    driver.find_element(By.ID, "reveal").click()

    # Wait for the content to be visible
    try:
        # The 'revealed' div becomes visible
        revealed_div = WebDriverWait(driver, 10).until(
            EC.visibility_of_element_located((By.ID, "revealed"))
        )
        print("Revealed content:", revealed_div.text)
    except Exception as e:
        print("Revealed content not found:", e)

    driver.quit()

if __name__ == "__main__":
    main()

Scraping Single-Page Apps

Single-Page Applications (SPAs) are websites that load a single HTML page and then dynamically update content as the user interacts. Navigation within an SPA often doesn't involve full page reloads.

Selenium handles SPAs well because it acts as a full browser. You interact with elements (like clicking navigation links) and Selenium executes the JavaScript, updating the DOM. You then apply the same waiting and extraction techniques.

Quick Check on Waiting

When scraping dynamic content that appears after the initial page load, which Selenium technique is crucial to ensure the content is available before attempting to extract it?

Recap: Dynamic Data Extraction

Great job! You've learned how to tackle dynamic web content:

Many sites use JavaScript for AJAX calls, DOM manipulation, and SPAs.
Traditional tools like Requests and BeautifulSoup cannot execute JavaScript.
Selenium, by controlling a full browser, can render JavaScript.
Explicit waits are essential to ensure dynamically loaded content is present before extraction.

This skill opens up a vast number of modern websites for your scraping projects!

常见问题解答

「从 JavaScript 中提取数据」课时是免费的吗？

是的 — 「从 JavaScript 中提取数据」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Web Scraping & Bots 课程的其余内容，请升级到 CoddyKit PRO。 Web Scraping & Bots 课程共包含 4 节课。

「从 JavaScript 中提取数据」这节课中我会学到什么？

学习获取由 JavaScript 生成或加载的数据，包括 AJAX 调用和单页应用中的内容。你通过在浏览器中直接运行的动手代码来练习 Web Scraping & Bots，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Web Scraping & Bots 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Web Scraping & Bots 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 3 节课，共 4 节。

「从 JavaScript 中提取数据」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Web Scraping & Bots 课中编写并运行代码吗？

能。每节 Web Scraping & Bots 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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