Automatic Item List Extraction

This repository is a temporary container for experiments in automatic extraction of list and tables from web pages. At some later point I will merge the surviving algorithms either in scrapely or portia.

I document my ideas and algorithms descriptions at readthedocs.

The current approach is based on the HTML code of the page, treated as a stream of HTML tags as processed by scrapely. An alternative approach would be to use also the web page rendering information (this script renders a tree of bounding boxes for each element).

Installation

pip install -r requirements.txt
python setup.py develop

Running

If you want to have a feeling of how it works there are two demo scripts included in the repo.

• demo1.py Will annotate the HTML code of a web page, marking as red the lines that form part of the repeating item and with a prefix number the field number inside the item. The output is written in the file 'annotated.html'.

  python demo1.py https://news.ycombinator.com
  

  

• demo2.py Will label, color and draw the HTML tree so that repeating elements are easy to see. The output is interactive (requires PyQt4).

  python demo2.py https://news.ycombinator.com
  

  

Algorithms

We are trying to auto-detect repeating patterns in the tags, not necessarily made of of li, tr or td tags.

Clustering trees with a measure of similarity

The idea is to compute the distance between all subtrees in the web page and run a clustering algorithm with this distance matrix. For a web page of size N this can be achieved in time O(N^2). The current algorithm actually computes a kernel and from the kernel computes the distance. The algorithm is based on:

Kernels for semi-structured data
Hisashi Kashima, Teruo Koyanagi

Once we compute the distance between all subtrees of the web page DBSCAN clustering is run using the distance matrix. The result is massaged a little more until you get the result.

Markov models

The problem of detecting repeating patterns in streams is known as motif discovery and most of the literature about it seems to be published in the field of genetics. Inspired from this there is a branch (MEME and Profile HMM algorithms).

The Markov model approach has the following problems right now:

• Requires several web pages for training, depending on the web page type
• Training is performed using EM algorithm which requires several attempts until a good optimum is achieved
• The number of hidden states is hard to determine. There are some heuristics applied that work partially

These problems are not unsurmountable (I think) but require a lot of work:

• Precision could be improved using conditional random fields. These could alleviate the need for data.
• Training can run greatly in parallel. This is actually already done using joblib in a single PC but it could be further improved using a cluster of computers
• There are some papers about hidden state merging/splitting and even an infinite number of states