Writing a scraper
This page outlines how to develop a scraper for apertium using our RFERL scraper. The code can be found in our subversion repository at https://svn.code.sf.net/p/apertium/svn/trunk/apertium-tools/scraper.
- Visit the website which you plan to scrape and locate the archive section which usually offers an interface to select a given day and see a list of links to articles published on that day.
- Familiarize yourself with the structure of the URL and how manipulating it will yield a different set of articles to scrape.
- Write a driver script named
scrp-*.pywhich will given a certain range of dates (or other parameters depending on the site's structure), be able to generate, for example, a list of tuples containing the article's link, its title and its publication date. LXML and BeautifulSoup are two useful tools for scraping HTML.
- Add an entry to the
scraper_classes.pywhich maps from the name of the website to a unique Scraper class.
- Define a new class in
scrapers.pythat inherits the Scraper class with two functions:
scraped(). Details are described below in the
scrapers.pysection. Again, LXML/BeautifulSoup will be very useful for scraping the article content.
- Finally, in the driver script loop through the list of articles and send each article to the Scraper class you created to fill the corpus with articles. Have a look at the various
scrp-*.pyscripts currently available to get a feel for how to use the Scraper class.
You need to define a new class in scrapers.py that inherits the Scraper class.
Your new class will have two new functions:
- This takes a url and converts it to a unique "article id". For sites that use some form of unique id for their articles (e.g., http://example.com/news?id=3141592653 or http://example.com/news/3141592653.html), you'll want to extract the id, probably with a simple regex. However, if this is for some reason not unique, or the site doesn't use unique ids, or if it's difficult to extract for some reason, it's okay to make a hash of the full url (which should be unique...). There are examples of both of these implemented in other scrapers in scrapers.py
- The first thing this function does is to fill
self.docwith the contents of the page, by calling
self.get_content(). This is all written for you already, so just call the function once and you're ready for the hard stuff.
- The hard stuff consists of getting a cleaned, text-only version of just the article content from the page. You'll have to first make sure you know which element in the page is going to consistently contain just the article content, and then extract that out with lxml. You'll then want to take that element and clean it with lxml (since there are scripts and stuff that can be in there too that could get in the output), and then get the
.text_content()of the element. An example of all this follows:
- The first thing this function does is to fill
self.get_content() cleaned = lxml.html.document_fromstring(lxml.html.clean.clean_html(lxml.html.tostring(self.doc.xpath("//div[@align='justify']")).decode('utf-8'))) cleaned = cleaned.text_content() return cleaned.strip()
If you are scraping RFERL content, you will need category names and numbers of only the real content categories.
Issues with newlines
Problem: The characters "& # 1 3 ;" (spaced apart intentionally) appear throughout after scraped content is written to .xml file.
Research: Retrieving the page html through using either
wget results in the problematic characters not appearing in final .xml output, however they reappear when the html is downloaded through a Python HTTPConnection. Since furthermore the characters are not present in other preceding output of the page html, it can be intelligently assumed that the error occurs with lxml:
lxml.html.document_fromstring(lxml.html.clean.clean_html(lxml.html.tostring(doc.find_class('zoomMe')).decode('utf-8'))). Directly following this step, the characters appear in the xml output. However, that still leaves uncertain the discrepancy between manually downloaded code and python downloaded code. This difference is likely due to
wget treating the code differently than python does. This can be painlessly confirmed with a
diff command which confirms that most (i.e. 95%) of the discrepancies are whitespace. The characters represent "\r", the carriage return. Online research shows that these problems can be attributed to Windows being incompatible with Linux\Unix standards: "When you code in windows, and use "DOS/Windows" line endings, the your lines will end like this "\r\n". In some xhtml editors, that "\r" is illegal so the editor coverts it to "& # 1 3"." Accordingly, running scrp-azzatyk.py shows that the offending characters unilaterally appear following the end of lines in the HTML.
Suggested Solution: The simplest solution is to manually remove the "\r" from raw html after download, like so:
res.read().decode('utf-8').replace('\r',' '). This should have no side effects for two reasons. One, HTML generally ignores conventional whitespace. Two, each "\r" is likely followed by a "\n", so replacing "\r" with nothing will only remove extraneous characters while otherwise preserving whitespace. This will solve the problem because the problematic characters represent "\r". This type of a solution to this seemingly not uncommon problem has been utilized by others and will ensure compatibility with Windows style "\r\n".This "solution" has been implemented.
Problem: The character "x" appears throughout after scraped content is written to .xml file.
Research & Solution: The problem was a small error due to not filtering out a bad class in ScraperAzattyk, the problem has been fixed and will be committed. This solution has been committed.
Problem: Paragraphs are not always being created correctly in scraped content, i.e. breaks tags are occasionally ignored
Research: Testing shows that the problem is occurring when two break tags are present on two separate lines and they are directly followed by another tag, generally an
em or a
strong, however the same problem has been observed with other tags. In the case that the break tags are seperated by text, lxml properly handles them. However, in the case that they are not, lxml fails to properly recognize the break tags. Test script, Test HTML
Suggested Solution: Submit a bug report to lxml. We could create custom Element classes? I'm fairly sure that even if we managed to do that, it would be fairly inelegant. A bug report has been filed.