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Web Scraping & Bots · レッスン

JavaScriptからのデータ抽出

AJAX呼び出しやシングルページアプリケーションのコンテンツなど、JavaScriptによって生成または読み込まれるデータを取得する方法を学びます。

「JavaScriptからのデータ抽出」はCoddyKit上の無料Web Scraping & Botsレッスンです。 これはレッスン3/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはWeb Scraping & Bots学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Web Scraping & Botsコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

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!

よくある質問

「JavaScriptからのデータ抽出」レッスンは無料ですか？

はい。「JavaScriptからのデータ抽出」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Web Scraping & Botsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Web Scraping & Botsコースには全4レッスンが含まれています。

「JavaScriptからのデータ抽出」で何を学びますか？

AJAX呼び出しやシングルページアプリケーションのコンテンツなど、JavaScriptによって生成または読み込まれるデータを取得する方法を学びます。 ブラウザで直接実行するハンズオンコードでWeb Scraping & Botsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Web Scraping & Botsを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのWeb Scraping & Botsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン3/4です。

「JavaScriptからのデータ抽出」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このWeb Scraping & Botsレッスンでコードを書いて実行できますか？

はい。すべてのWeb Scraping & Botsレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. Selenium入門
  2. ブラウザー操作の自動化
  3. JavaScriptからのデータ抽出
  4. 動的ページの待機戦略
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