- Web Scraping With Go
- Web Scraper Golang
- Web Scraping With Golang Programming
- Web Scraping Golang Vs Python
In general programming interfaces are contracts that have a set of functions to be implemented to fulfill that contract. Go is no different. Go has great support for interfaces and they are implemented in an implicit way. They allow polymorphism in Go. In this post, we will talk about interfaces, what they are, and how they can be used.
Web scraping with chromedp and golang throws message “could not retrieve attribute outerHTML encountered exception Uncaught”. Web scraping to fill out (and retrieve) search forms? Python web scraping involving HTML tags with attributes. Scrape web page and retrieve javascript variables.
What is an Interface?
- Go is a programming language built to resemble a simplified version of the C programming language. It compiles at the machine level. Go was created at Google in 2007 by Robert Griesemer, Rob Pike, and Ken Thompson.
- In general programming interfaces are contracts that have a set of functions to be implemented to fulfill that contract. Go is no different. Go has great support for interfaces and they are implemented in an implicit way.
- Problems while web scraping with golang. Ask Question Asked 20 days ago. Active 20 days ago. Viewed 64 times 0. I'm creating a web scraper with golang and I just wanted to ask some questions about how most of them work. For example, how does Googlebot not use a lot of bandwidth when scraping because you have to go to each URL to get data.
- The first step to web scraping is being able to make an HTTP request. Let's look a very basic HTTP GET request and how to check the response code and view the content. Note the default timeout of an HTTP request using the default transport is forever. // makehttprequest.go.
An interface is an abstract concept which enables polymorphism in Go. A variable of that interface can hold the value that implements the type. Type assertion is used to get the underlying concrete value as we will see in this post.
Declaring an interface in GoLang
An interface is declared as a type. Here is the declaration that is used to declare an interface.
type interfaceName interface{}
Zero-value of an interface
The zero value of an interface is nil. That means it holds no value and type. The code below shows that.
The empty interface in Go
An interface is empty if it has no functions at all. An empty interface holds any type. That’s why it is extremely useful in many cases. Below is the declaration of an empty interface.
var i interface{}
Implementing an interface in GoLang
An interface is implemented when the type has implemented the functions of the interface. Here is an example showing how to implement an interface.
Implementing multiple interfaces in Go
Multiple interfaces can be implemented at the same time. If all the functions are all implemented then the type implements all the interfaces. Below the type, the bird type implements both the interfaces by implementing the functions.
Composing interfaces together
Interfaces can be composed together. The composition is one of the most important concepts in software development. When multiple interfaces are implemented then the type has performed composition. This is really helpful where polymorphism is needed.
Values in an interface
Interface values have a concrete value and a dynamic type.
In the code above chirper is of type Bird but has a concrete value of {Chirpir}.
Type assertion using the interface
Type assertion is a way to get the underlying value an interface holds. This means if an interface variable is assigned a string then the underlying value it holds is the string. Here is an example showing how to use type assertion using interfaces.
Type switch using an interface
Type switches are an extremely similar control structure like the switch-cases, the only difference is here the interface type is used to switch between different conditions.
Equality of interface values
The interface values can be equal if any of the conditions shown below are true.
- They both are nil.
- They have the same underlying concrete values and the same dynamic type.
Using interfaces with functions
Interfaces can be passed to functions just like any other type. Here is an example showing the usage of the interface with functions. A great advantage when using an interface is that it allows any type of argument as we can see in this code below.
Uses of an interface
Interfaces are used in Go where polymorphism is needed. In a function where multiple types can be passed an interface can be used. Interfaces allow Go to have polymorphism.
Interfaces are a great feature in Go and should be used wisely.
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In this article we’re going to have a look at how to mock http connections while running your tests on your golang application.
Since we do not want to call the real API or crawl an actual site to make sure that our application works correctly, we want to make sure that our application works within our defined parameters.
There's a great module that can help us with the task of mocking HTTP responses for tests called httpmock
HTTP mocks for web scraping
Let's say we have a component in our application that will do some web scraping, so we might use something like goquery.
In the below example we'll use a simple function that visits a website and extracts the content of the <title> tag.
filename: scrape.go
Now if we are to write a unit test for that, we can do that as follows:
filename: scrape_test.go
In the test we run the function and compare the title we expect with the title that was scraped by the function.
Now the problem with this test is, that when ever we run go test it will actually go to my website and read the title. This means two things:
- Our tests will be slower and more error prone than they could be
- I can never change my website title without changing the tests for this project
- Most important: We introduced a dependency outside our control for our program that doesn't have any relation to it
To fix this we commonly use mocks, a way of faking http responses, but to actually have the exchange of information happen on the computer where the tests are run, without having to rely on an external webserver or API backend to be available.
HTTP mocks for API requests
In Golang we can use httpmock to intercept any http requests made and pin the responses in our tests. This way we can verify that our program works correctly, without having to actually send a requests over the network.
To install httpmock we can add a go.mod file:
and running go mod download.
Rewriting our scrape_test.go would look like this:
after which we can run go test and it should produce the following output:
Let's go over the most important changes ot the file:
myMockPage :=...sets up our example response, a piece of plain text that our function will parse into a HTML and look for the titlehttpmock.Activate()activates the mocking, before this no requests can be interceptedhttpmock.RegisterResponder()defines the METHOD and the URL, so GET or POST and an address at which we fake an http responsehttpmock.NewStringResponderwill need a status code and a string to respond with instead of what actually lives at that URLhttpmock.DeactivateAndReset()stops mocking responses for the rest of the test
If you instead want to mock an API response you can use something like this:
That's it! Our client consuming the string should take care of the JSON parsing.
If you're familiar with mocking http connections in node.js you may have heard of the nock library, which is pretty popular when building JavaScript projects.
Web Scraper Golang
Hope you enjoyed this little post about mocking in GO, let me know what you're building in the comments!
Web Scraping With Golang Programming
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Web Scraping Golang Vs Python
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