web-scraping（网页抓取）

web-scraping

Reliable scraping patterns for static HTML and JavaScript-rendered pages.

Overview

Start with requests + BeautifulSoup for static pages. When content is rendered by JavaScript, switch to Playwright. Always use timeouts, retries, and explicit user-agent headers.

When to Use

• Extracting structured data from HTML pages
• Monitoring content changes
• Handling SPAs or dynamic content with a headless browser

When Not to Use

• The site forbids scraping in ToS/robots.txt
• An official API is available (prefer it)

Quick Start

uv pip install requests beautifulsoup4

import requests
from bs4 import BeautifulSoup

resp = requests.get("https://example.com", timeout=10)
resp.raise_for_status()

soup = BeautifulSoup(resp.text, "html.parser")
print(soup.title.text)

Core Patterns

1. Use a Session: reuse connections and set default headers.
2. Always set timeouts: avoid hanging requests.
3. Respect robots.txt/ToS: throttle and avoid sensitive paths.
4. Stable selectors: prefer semantic attributes or ids.
5. Capture HTML snapshots: store responses for debugging.
6. Escalate to Playwright: for JS-rendered pages.

Requests Patterns

import requests
from requests.adapters import HTTPAdapter
from urllib3.util import Retry

session = requests.Session()
session.headers.update({"User-Agent": "my-scraper/1.0"})

retries = Retry(total=3, backoff_factor=0.1, status_forcelist=[502, 503, 504])
session.mount("https://", HTTPAdapter(max_retries=retries))

resp = session.get("https://example.com", timeout=10)
resp.raise_for_status()

BeautifulSoup Patterns

from bs4 import BeautifulSoup

soup = BeautifulSoup(html, "html.parser")

# CSS selectors
links = soup.select("a[href]")
main = soup.select_one("#main")

Playwright (Dynamic Pages)

from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    page = browser.new_page()
    page.goto("https://example.com", wait_until="domcontentloaded")
    page.wait_for_selector(".product")
    html = page.content()
    browser.close()

Troubleshooting

• Blocked requests: set User-Agent, add backoff, reduce rate.
• Empty HTML: page is JS-rendered; use Playwright.
• Selector breakage: update CSS selectors to stable attributes.

References

• https://docs.python-requests.org/en/latest/
• https://www.crummy.com/software/BeautifulSoup/bs4/doc/
• https://playwright.dev/python/