What are the common pitfalls when using Scraper (Rust) for web scraping?

Scraper is a web scraping library for Rust that provides a simple interface for parsing and querying HTML documents using CSS selectors. While it's a powerful tool, users can encounter several common pitfalls when using Scraper for web scraping:

1. Incorrect CSS Selectors: One of the most common issues when using Scraper is using incorrect or outdated CSS selectors. Websites often change their structure, rendering previous selectors obsolete. Always verify that your selectors match the current structure of the web page.

2. Handling Dynamic Content: Scraper, like many scraping libraries, is not designed to handle JavaScript-generated content. If the content you're trying to scrape is loaded dynamically with JavaScript, Scraper won't be able to see it without the help of additional tools like headless browsers (e.g., Headless Chrome or Firefox).

3. Rate Limiting and Bans: Aggressive scraping can lead to your IP address being banned by the website. It's important to respect the website's robots.txt file and to implement polite scraping practices such as rate limiting and rotating user agents.

4. Error Handling: Network issues, HTTP errors, and parsing errors can occur during web scraping. Proper error handling is crucial to ensure the scraper can recover or at least fail gracefully.

5. Poor Performance with Large Documents: When dealing with very large HTML documents, parsing and querying with Scraper can be resource-intensive and slow down the scraping process. It's important to optimize your queries and only load and parse the necessary parts of the document.

6. Character Encoding Issues: Web pages can use a variety of character encodings. If the encoding is not correctly handled, it can lead to garbled text in the scraped data. Users should make sure to correctly detect and handle the character encoding of the pages they're scraping.

7. Incomplete Documentation: As with many open-source projects, documentation might not cover all use cases or might be outdated. Users often have to rely on examples, community support, or directly reading the source code to understand how to use certain features.

8. Legal and Ethical Considerations: Web scraping can be legally and ethically controversial. It's important to understand and respect the legal implications of scraping a particular website and to obtain proper permission if necessary.

Here is an example of how to use Scraper in Rust for a simple scraping task, along with code to handle some of the issues mentioned above:

extern crate reqwest;
extern crate scraper;

use scraper::{Html, Selector};
use std::error::Error;

fn main() -> Result<(), Box<dyn Error>> {
    // Make an HTTP request to retrieve the HTML content of the page
    let res = reqwest::blocking::get("https://example.com")?.text()?;

    // Parse the HTML document
    let document = Html::parse_document(&res);

    // Create a CSS selector to target the desired elements
    let selector = Selector::parse(".some-class").unwrap();

    // Iterate over elements matching the selector
    for element in document.select(&selector) {
        // Extract the text from each matched element
        let text = element.text().collect::<Vec<_>>().join(" ");
        println!("Found text: {}", text);
    }

    Ok(())
}

In this example, we've made an HTTP GET request to https://example.com using reqwest, a popular HTTP client for Rust. We then parse the returned HTML using Scraper and query it with a CSS selector. Proper error handling is implemented with Rust's Result type.

It's important to note that the above code does not include any rate limiting, polite scraping practices, or dynamic content handling, which would be necessary for a robust and respectful scraping tool.