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Python web crawler is a program written in Python that automatically accesses web pages, parses html or json data, and extracts the required information. Some important knowledge points related to Python web crawlers will be introduced in detail below.
1. Python basic syntax:
Variables and Data Types: Learn how to declare variables and common data types in Python such as numbers, strings, lists, dictionaries, and more.
- Conditional statements and loop statements: master if statements, for loops, and while loops, which are used for conditional judgment and loop execution of code blocks. 2. Functions and modules: Learn how to define and use functions, and how to use Python modules (libraries) to extend functions 3. File operations: Learn how to read and write files, which can be used to store and process crawler data.
2. HTML network structure:
- HTML basics: Understand the basic tags of HTML (such as <html>, <head>, <body>, etc.), understand the nesting relationship of tags and the use of attributes.
- CSS selector: Master the positioning of web page elements through CSS selectors. In crawlers, third-party libraries such as BeautifulSoup and lxml can be used to parse HTML and provide flexible and powerful CSS selector functions.
3. Data in JSON format:
- JSON basics: Understand the basic syntax and data structure of JSON (JavaScript Object Notation), including objects, arrays, key-value pairs, etc.
- JSON Parsing: Learn how to use Python's built-in json module to parse and process JSON data, converting it into Python objects for manipulation.
4. Crawler process:
- Initiate HTTP request: Use third-party libraries in Python (such as Requests, urllib) to send HTTP requests to obtain web page content.
- Parsing HTML or JSON: Use third-party libraries (such as BeautifulSoup, lxml, json) to parse HTML or JSON data and extract target information.
- Data processing and storage: To process and clean the extracted data, you can use Python's built-in string processing method, and then store the data in a file or database.
- Anti-crawler and restrictions: Understand the anti-crawler mechanism and master the methods to bypass common restrictions, such as setting request header information, using proxy IP, processing verification codes, etc.
5. Practical cases:
- Crawling webpage content: Use the Requests library to send HTTP requests to obtain webpage content, and use BeautifulSoup or lxml to parse HTML and extract the required information.
- Parsing JSON data: read files containing JSON format data or obtain JSON data through HTTP requests, use Python's json module to parse the data and perform operations.