What is a web crawler? Can you give us a more detailed introduction?

A web crawler (also known as a web spider, web bot, in the FOAF community, more often called a web chaser), is a program or script that automatically scrapes information from the World Wide Web according to certain rules. Other names that are not commonly used are ants, auto-indexes, emulators, or worms.

Let's analyze what core work web crawlers do:

Send a request over the network to the specified URL to get the server response.

Use some kind of technique (such as regular expressions, xpath, etc.) to extract the information we are interested in from the page.

Efficiently identify the link information in the response page and follow those links recursively to perform the steps described here;

Use multithreading to effectively manage network communication interactions.

If you directly use the built-in urllib and re modules of python, can you write your own web crawler? The answer is yes, it's just more complicated. It's like we're going from Guangzhou to Shaoguan, can we go on foot?

The answer is yes, it's just more troublesome.

Let's move on to the core work of a web crawler:

Send a request to the URL to get the content of the server response. This core job is actually the general work that all web crawlers need to do. Generally speaking, general work should be implemented by a crawler framework, which can provide more stable performance and higher development efficiency.

Extract the information we are interested in from the page. This core job is not generic! The information of interest to each project can be different, but it is very inefficient to use regular expressions to extract information because regular expressions are designed primarily to process textual information, and HTML documents are not only text documents, but also structured documents, so it is not appropriate to use regular expressions to process HTML documents.

It is much more efficient to extract information using Xpath. Identify the link information in the response page. This core work can be done with regular expressions, but it's too inefficient and would be more efficient with xpath.

Multithreaded management: This core work is generic and should be done by the framework.

As long as there is something on the web page, it can be crawled down by crawling through crawlers.

In general, a Python crawler requires the following steps:

Locate the URL of the web page that needs to crawl the content

Open the check page of the web page (i.e. view the html** and press the F12 shortcut to enter).

Find the data you want to extract in HTML

Write python** to request and parse web pages

Storing data

Of course, knowing python is the premise, and it is not easy for Xiaobai to learn by himself, it takes a considerable amount of time to adapt to the grammar and logic of python, and he must insist on knocking ** by himself and keep practicing.

If you are not confident in yourself, you can also consider watching programming courses and learning at the teacher's pace, so that you can master the python grammar system relatively quickly and get sufficient case exercises.

The kind of insect that either has long legs or crawls is called a reptile. Reptiles should be divided into those that can fly, and those that can't fly, anyway, there are many types, that is, crawling, and there are really a lot of them. If there is no such thing as a crawler, they should not be called a crawler.

Insects that crawl on the ground, or on vegetable leaves, or fruits and vegetables are called reptiles!

Reptiles are crawling insects that are generally smaller.

1.Logically it means and (and). A & b means that both elements a and b are indispensable.

2.&& can also be used as a bit operator, when the expression on both sides of the & operator is not of boolean type, & means bitwise and operation, we usually use 0x0f to do & operations with an integer to get the lowest 4 bits of that integer, for example, 0x31 & 0x0f result is 0x01.

It was originally a concatenation of the Latin et (meaning and). The earliest & much like the combination of e and , with the development of printing technology, this symbol gradually formed its own style and detached from its original shadow. In English, it stands for and, and can also be pronounced as "z".'da”。

Extended Materials. AND && can be used as operators for logical and (and), when the result of the expression on both sides of the operator is true, the whole result is true, otherwise, as long as one of the parties is false, the result is false.

&& can also be used as a bit operator, when the expression on both sides of the & operator is not of boolean type, & means bitwise and operation, we usually use 0x0f to do & operations with an integer to get the lowest 4 bits of that integer, for example, 0x31 & 0x0f result is 0x01.

Crawlers are also called web chasers.

It is a program or script that automatically scrapes information from the World Wide Web according to certain rules.

How it works: Traditional crawlers start from the URL of one or more initial web pages, obtain the URL on the initial web page, and then continuously extract new URLs from the current page and put them in the queue until a certain stop condition of the system is met.

The cost of writing a crawler program is too high, so you can choose crawler software.

What is a Crawler? A reptile is a crawling insect that crawls on the ground.

Crawler technology is a type of automated program.

Crawler is an automated program that can scrape data information from web pages and save it, its principle is to simulate a browser to send a network request, accept a request response, and then automatically scrape Internet data according to certain rules.

Search engines crawl from one crawler to another through these crawlers, tracking links in web pages and visiting more web pages, a process called crawling, and these new crawlers are stored in a database waiting to be searched. In a nutshell, a crawler is all about accessing the internet without interruption, and then taking the information you specify from it and returning it to you. On our Internet, there are countless crawlers crawling data and returning it to users.

The functionality of crawler technology

1. Get the web page

Getting a web page can be simply understood as sending a network request to the server of the web page, and then the server returns to the source of our web page**, where the underlying principle of communication is more complex, and Python encapsulates the urllib library and requests library for us, etc., which can make us very simple to send various forms of requests.

2. Extract information

The source code of the obtained web page contains a lot of information, and if you want to extract the information we need, you need to further filter the source code. You can choose the RE library in Python to extract information in the form of regular matching, or you can use the beautifulsoup library (bs4) and other analytical sources**, in addition to the advantages of automatic coding, the bs4 library can also structure the output of source ** information, which is easier to understand and use.

3. Save data

Once we have extracted the useful information we need, we need to save it in Python. You can save it as text data through the built-in function open, or you can save it as other forms of data with a third-party library, for example, you can save it as common xlsx data through the pandas library, and if you have unstructured data such as **, you can also save it to an unstructured database through the pymongo library.

1. Web crawlers, also known as web spiders, web robots, in the FOAF community, more often called web chasers, are programs or scripts that automatically crawl information from the World Wide Web according to certain rules, and some other names that are not commonly used are ants, automatic indexes, simulators or worms.

2. Most crawlers follow the process of "sending a request - obtaining the page - parsing the page - extracting and storing the content", which is actually a process that simulates the process of using the browser to obtain web page information.

3. To put it simply, the crawler is a detection machine, and its basic operation is to simulate human behavior to go to each ** walk, click the button, check the data, or recite the information you see. It's like a bug crawling tirelessly around a building.

4. It can be simply imagined: every reptile is your "doppelganger". It's like Sun Wukong plucking a pinch of sweat hair and blowing out a bunch of monkeys.

Reptiles, vertebrates. Also known as reptiles and reptiles, amniotic animals belonging to the quadruped class, are the common name for all species of sauropids and zygomorphs except birds and mammals, including turtles, snakes, lizards, crocodiles, extinct dinosaurs and mammal-like reptiles, etc.

Skeletal systemThe skeletal system of reptiles is mostly composed of hard bones, which are highly ossified and rarely retain cartilage parts.

Most reptiles lack secondary jaws, so when they eat, they can't breathe at the same time. Crocodiles have developed bony secondary jaws that allow them to breathe continuously while semi-submerged in the water and prevent their prey from injuring the brain when the prey in their mouths struggles. Skinks also evolved bony secondary jaws.