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Web Crawler Intro

What is a Web Crawler?

A Web Crawler (also known as a Web Scraper or Spider) is a script or program that automatically navigates websites and extracts or uploads data according to predefined rules. It simulates browser behavior to perform actions like clicking links, downloading articles, or extracting specific information from web pages.

Crawlers help automate repetitive web tasks and are widely used in data engineering and analysis.

Crawler Behavior


Why Use a Web Crawler?

The internet contains vast amounts of data that's hard to collect manually. Web crawlers are essential for:

  • Search engine indexing (e.g., Google)
  • Collecting article lists from news websites
  • Gathering job listings from HR platforms
  • Price comparison across e-commerce sites
  • Investment data analysis
  • Academic research and data collection

They are especially useful when extracting large volumes of similarly structured web pages.


How It Works: Simulating Browser Behavior

When you visit a website using a browser, it sends an HTTP request to a remote server, which returns the HTML source code. A crawler mimics this process:

  1. Sends an HTTP request (e.g., GET)
  2. Receives a response (HTML/JSON/XML)
  3. Parses the data structure
  4. Extracts, cleans, and stores the necessary information

Static vs Dynamic Pages

TypeCharacteristicsHandling Method
Static PageData is directly embedded in HTML sourceUse requests + BeautifulSoup
Dynamic PageData is generated by JavaScript on the pageUse selenium or Playwright to simulate browser

Common Tools and Libraries

Tool / LibraryPurpose
requestsSends HTTP requests
BeautifulSoupParses HTML content
lxml / html.parserParsing engines for HTML
seleniumSimulates browser interaction for dynamic pages
pandasOrganizes and stores structured data
re (Regex)Text processing and pattern extraction

Example

Crawler example