Basic Web Crawler written in Python

List of files:

1. web-crawler.py - Main program file containing the crawler, the parser and a method to fetch results from google. This file when executed crawls the web based on a query passed by the user

2. ranking_function.py - This file is used to calculate the ranking function of a given webpage provided a query. The ranking function depends on how much the webpage is relevant to the given query.

3. customurllib.py - Overriden urllib library for handling http_401 / Unauthorized access error.

Execution:

To execute the program install all the libraries listed in the requirements.txt file. After complete installation run the file web-crawler.py. The program will then ask the user for a query. Based on that query the crawler will crawl the web and write the results in a file named output.txt

Known Bugs:

1. The program, for some webpages throws a UnicodeEncodeError where it has trouble parsing some unicode to utf-8 while calculating BM25 score and other places. I am unsure what MIME type is causing this error and I am unable to recreate it.

Output file pattern:

Pattern = "URL | BM25 score | depth | return code | time"

Execution:

1. When the user runs the file web-crawler.py the program asks the user for a search query for focused crawling.

2. The program then creates an Instance of the class WebCrawler with this query. The creation of the Instance leads to the calling of the method WebCrawler.fetch_google_results().

3. The google results are then stored in a priority queue along with their depth and Okapi BM25 score.

4. The Program then calls the method WebCrawler.crawl(). This method retrieves the url with the highest BM25 score from the Priority Queue and then parses it by calling WebCrawler.parse_page() method alongside adding the URL to visited list.

5. The parse page method extracts all the links from the given webpage, calculates the BM25 score for the page and inserts the page into the URLs priority queue.

6. Once all links have been parsed we jump back to step 4.

7. After the required no. of links have been crawled, the output is written to a file output.txt