Mastering Scrapy: How to Correctly Implement Deny Rules in Your Crawl Spider

Learn how to effectively apply deny rules in Scrapy's Crawl Spider by addressing common pitfalls and mastering regular expressions.
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This video is based on the question https://stackoverflow.com/q/62932886/ asked by the user 'Armin Abele' ( https://stackoverflow.com/u/13889939/ ) and on the answer https://stackoverflow.com/a/62936741/ provided by the user 'Lou Franco' ( https://stackoverflow.com/u/3937/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Scrapy: How to Correctly Implement Deny Rules in Your Crawl Spider

As a beginner in web scraping with Scrapy, encountering issues with filter rules can be frustrating. One common problem that newcomers face is the deny rules used in Crawl Spiders. In this post, we'll address a specific issue regarding deny rules, especially the use of regular expressions, and how to fix it effectively.

The Problem: Deny Rules Not Working

A user trying to filter out URLs containing the word "versicherung" and a double ? structure faced challenges getting Scrapy to respect their deny rule. The initial code used was:

[[See Video to Reveal this Text or Code Snippet]]

However, it didn't filter the URLs correctly, allowing unwanted links to creep in. Let’s dive into solving this problem.

Understanding Deny Rules

What Are Deny Rules?

Deny rules in Scrapy allow developers to specify patterns for URLs that should not be crawled. This is especially useful when you want to avoid scraping certain unwanted URLs that could clutter your results.

Common Syntax Issues

The syntax you use in deny rules must adhere to regular expression (regex) standards. Here are some key clarifications on regex syntax for this case:

. represents any character.

means zero or more of the preceding character or expression.

\ is used to escape a special character but shouldn't be used with * in your case, since it negates its function.

The Solution: Correct Syntax for Deny Rules

After analyzing the original deny rules, the following adjustments are necessary:

Modify the regex for the word "versicherung":

The original regex r'*versicher*' should be transformed to r'.*versicher.*'.

Optionally, based on Scrapy's documentation, you can simplify your regex to just r'versicher' since it efficiently captures any URL containing that word.

Addressing the double ? structure:

The expression for detecting double question marks should be corrected to r'.*??.*' or simply r'??' if you're looking to catch the double question mark directly.

Revised Example of Rules

Here’s how you could write the corrected rules:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

In conclusion, ensuring that your regex is correctly formatted is crucial for the effective use of deny rules in Scrapy. By understanding how to use regex symbols properly, you can filter out unwanted URLs seamlessly.
If you encounter similar issues, always verify your syntax and test your expressions. Happy scraping!