I go into further detail in the exception-handling chapter. We wrap that in a begin block. The rescue block is where the program will jump to if an error happens in the begin block. The else block is what should happen if no error is encountered in the begin block.
Finally, the ensure block will always execute no matter what happens in the begin block. I may cover error-handling in a later edition. For now, the Pragmatic Programmer's Guide does a nice job of it. The specified local directory should have about new HTML files in it.
This is the sample output to screen:. Astute readers will note that my code does little more than mass download every page on the Nobel laureates page. This is something a plugin like DownloadThemAll! The scraper doesn't actually get data. This is true. In the above code snippets, I only show the basics of doing a web-crawl. With a little more configuration, we can use the structure of the Nobel laureates table to structure the data from each individual page to our liking.
Such as finding the nationalities of every Chemistry prize winner, to use a cliche example. I will do a write-up of this in the near-feature iteration of this lesson. But by now, you've learned enough to do this on your own.
You may not have data yet, but you have the material from which the data can be easily extracted. Spidering a website, link by link, as we did in the Nobel prizes Wikipedia example , will work for most websites. However, it can be kind of tedious to examine each different kind of page to figure out the link structure.
But if you do a little scouting and experimentation, you may find a pattern in the site's URL that you use to save yourself a considerable amount of time. The most obvious examples are sites that paginate their information. The federal government's Data. Clicking on the link for Page 2 predictably gets the page for the next 25 results. Take a look at the URL in your browser:. And if you visit Page 3 , the link looks like this:. See a pattern? We can simply write a loop that increments the page number in the URL.
We'll save each HTML page to disk, which we can later parse at our leisure. The last page number is helpfully included in the href attribute of the » button. So parse the first page with Nokogiri to find that number.
Then use a for loop to iterate through each page number:. By virtue of having scraped the Data. The limit parameter apparently tells the server the maximum number of results to send back per page.
The default limit, as we've seen, is So increasing the value for limit reduces the number of pages we need to loop through. Play around with it some more. You'll see that giving it an arbitrarily large number such as won't have any effect. Once you find a maximum, change the URL in the previous scraping script to include a set limit.
I was able to get the number of loop iterations to under How did I find this out? Just dumb luck from having previously scraped this site. The current design doesn't reveal the limit parameter so I just tried it out for fun, and it turns out the site designer didn't close down the use of that parameter. In most cases, blind experimentation won't net you much.
But don't underestimate the carelessness that can creep into a site design. In the following sections, I will show how your web inspector can help you suss out shortcuts and loopholes that aren't out in the open.
Here's a variation to the above change-the-page-number formula:. Take note of the URL :. That seems like a pretty obvious pattern. However, other small numbers seem to work for contractid , such as 10 :. So we can still loop through contract numbers. But it'll require a few modifications.
Here's a working script:. So we end up downloading two pages when we had all the info we needed in the first request step 2 from above.
We can skip trying to download it — and the DoD will appreciate our scraper not chewing up needless bandwidth. If the response's code is a , then its body method will contain the HTML we want:.
Otherwise, it gets us the webpage just as well as the other, higher-level methods. Every kind of website we've dealt so far involves pages with actual direct links sometimes known as permalinks. If you wanted to email someone: "Hey, check out these Oct. However, many websites require you to fill out and submit a form.
The website then directs you to a URL for a page of results. But the page at that URL depends on parameters set by that previous form. That URL does not act as a direct link. If you go to that URL directly, the page will not show you the same results. In essence, the results page depends on the user having first visited and filled out the previous form page.
Note: It's probably a massive drain for both FEC. This is just a proof of concept. One of its datastores includes the filing reports — as PDFs — of every campaign and committee.
I don't know; it's possible every single bit of information in the PDFs is compiled in a raw database somewhere else. Also, many of the PDFs contain poor-quality scans and handwritten answers. Parsing them is not a trivial task. The code used here can be used on similar websites of interest. Click Get Listing and you'll go to a new page with the results:. It's the address of a script that depends on the previous form being submitted. If you visit the link directly , you'll see this error page:.
Return to the search form page. Pop open your inspector's network panel review the chapter on the network panel if you need to , submit a search term, and check out the headers for a file named fecimg :. As you can see in the highlighted area bove, the search form makes what is called a POST request , which is a way for forms to submit a web request when the parameters can't fit in a URL. The Wikipedia API call for suggested search terms covered in the network panel chapter , for example, is able to fit its parameters in the URL:.
How do we see the parameters set by the FEC's image search form? The network panel reveals the parameters in the request headers. The following image is a close crop of the above photo, but cropped to the Form Data section this is using Chrome's inspector :.
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