Uncovering how web crawlers work

If the Internet is compared to a big web, then the web crawler is the "spider" crawling on this big web. The web page is like a node of the network, crawling to this is equivalent to visiting the website page, access to its information; The link relationship between web pages and web pages can be regarded as the connection between different nodes. After crawling through a node, the crawler can continue to crawl along the node connection to the next node, and continuously climb the subsequent web pages, so as to achieve the data acquisition of all relevant websites. Next, let's take a look at how crawlers work.


1. Get the Web page

Getting web pages is the first step of web crawler work, and it is the most critical link for a complete crawler work. In this step, the crawler needs to simulate a real request, send the access request to the server of the target site, and receive the response content returned from the server, that is, the source code of the web page. This process is similar to how we use a browser to access a website, but in crawlers, all the operations are automated and done by code.

In order to achieve web page acquisition, it is very convenient to use third-party libraries in Python crawler, of which requests and urllib are commonly used tools. These libraries provide simple yet powerful capabilities that enable users to easily simulate HTTP requests and transform access requests and responses into data structures suitable for processing.

When using the requests library, users can simulate different types of requests by constructing HTTP request headers, adding request parameters, and setting cookies. For example, a GET request is used to get the content of a web page, while a POST request is used to submit form data or interact with a server. After sending a request, the requests library waits for a response from the server and returns the content of the response to the user in the form of text or binary data.

urllib, on the other hand, is also a module in the Python standard library for web requests, which provides similar functionality to enable users to obtain web pages and download data. Although urllib is relatively simple to use, there are some scenarios where you may need to write more code.

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No matter which library is used, the goal of obtaining a web page is to obtain the source code of the web page for subsequent content parsing and data extraction. After the crawler obtains the web page, it can analyze the structure of the web page and extract the required information from it. In this way, the web crawler can help us quickly and accurately obtain the data we need in the massive information, and achieve the goal of automatic data collection.

2. Extract information

Extracting information is a very important link in the work of crawler. After obtaining the information content of the web page, it is necessary to analyze it and extract the data required by users. In general, the use of regular expression extraction is a more general method, but for more complex web page structure, the construction of regular expression is easy to error. At this time, third-party libraries such as lxml, pyquery, Beautiful Soup, etc. can be used to complete the extraction of information. These libraries present certain rules according to the structure of web pages to help users extract web page information more quickly and efficiently.

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3. Save data

After successfully extracting the information, the user needs to store the data and store it in a certain space. In fact, data can be saved in many forms, such as in a database, or simply as JSON text or TXT text. Users can choose the data storage method that suits their needs, such as MySQL and MongoDB databases, and can also save data to remote servers.

The operation principle of web crawler is an iterative process, through continuous request, analysis and storage, to achieve the collection and sorting of a large amount of information on the Internet. Web crawlers play an important role in various industries, from search engine search results to commodity price comparison, web crawlers provide us with convenient data access and application. However, it is worth noting that the use of web crawlers requires compliance with relevant laws and regulations and the site's usage agreement, ensuring the legality and ethics of the crawling process.

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