Here’s a change of pace. Our first few lessons focused on how you can use Python to goof with a bunch of local files. This time we’re going to try something different: using Python to go online and screw around with the Web.
Whenever I caucus with aspiring NICARians and other data hungry reporters, it’s not long before the topic of web scraping comes up. While automated text processing and database management may sound well and good, there’s something sexy about pulling down a fatty government database that catches people’s imagination and inspires them to take on the challenge of learning a new programming language. Or at least entertain the idea until they run into a road block.
A number of fellow travelers do a noble job instructing people on the basics during NICAR’s annual seminars. But scraping seems like such a sought-after skill that it feels like a good idea to throw up a basic walkthrough here, where beginners can cut and paste code and any feedback can be memorialized.
But before we get going, let me just say that I’m going to assume you read the first couple recipes and won’t be working too hard to explain the stuff covered there. And keep in mind that my keystrokes are coming right off my home computer, which runs Ubuntu Linux. I’ll try to provide Mac and Windows translations as we go, but I might muck a phoneme here and there. If anything is screwed up and doesn’t work on your end, just shoot me an email or drop a comment. We’ll iron it out.
Formalities aside, here’s the example task I’ve selected to achieve our mission.
- Install the necessary Python modules, mechanize and Beautiful Soup.
- Train our computer to visit Ben’s list of The Greatest Albums in the History of 2007.
- Parse the html and scrape out Ben’s rankings.
- Click through to Ben’s list of The Greatest Albums in the History of 2006 and repeat the scrape.
- Do it all over again, but this time download the cover art.
1. Download the mechanize and Beautiful Soup modules. Install them.
There are a dozen different methods for going about our task, so you shouldn’t assume the one I’m about to show you is the only or the best. It’s just one way to do it. And doing it this way requires a couple additions to your Python installation, which might seem a little daunting but should be doable unless IT has your computer on double secret probation.
A module is a collection of functions, defintions and statements contained in a separate file that you can import into your script. Examples native to Python used in our earlier scripts included “re”, “os” and “string.”
Out there on the Web, kind and ambitious programmers are constantly drafting, updating and publishing new modules to boil down complicated tasks into simpler forms. It it wasn’t for these people, praise be upon them, I probably wouldn’t have a job.
If you want to take advantage of their contributions, you need to plug their creations into your local Python installation. It’s usually not that hard, even on Windows!
To accomplish today’s task, we’re going to rely on two third-party modules. The first is mechanize, a Python translation of the popular Perl module for calling up and walking through Web pages. The second is Beautiful Soup, a superlatively elegant means for parsing HTML and XML documents. Working hand-in-hand, they can accomplish most simple web scrapes.
If you’re working Linux or Mac OS X, this is going to be a piece of cake. All you need is to use Python’s auto-installer Easy Install to issue the following commands:
sudo easy_install mechanize
sudo easy_install BeautifulSoup |
And now you can check if the modules are available for use by cracking open your python interpreter…
…and attempting to import the new modules…
from mechanize import Browser
from BeautifulSoup import BeautifulSoup |
If the interpreter accepts the commands and kicks down the next line without an error, you know you’re okay. If it throws an error, you know something is off.
I don’t have a lot of Python experience working in Windows, but the method for adding modules that I’ve had success with is simply downloading the .py files to my desktop and dumping them in the “lib” folder of my Python installation. If, like me, you use Activestate’s ActivePython distribution for Windows, it should be easily found at C:/Python25/lib/. And when you browse around the directory, you should already see os.py, re.py and other modules we’re already familar with. So just visit the mechanize and Beautiful Soup homepages and retrieve the latest download. Dump the .py files in your lib folder and now you should be able to fire up your python interpreter just the same as above and introduce yourself to our new friends.
With that out of the way, we now have all the tools we need to grip and rip. So let’s do it!
2. Open the command line, create a working directory, move there.
We’re going to start the same way we did in the first three lessons, creating a working folder for all our files and moving in with our command line.
cd Documents/
mkdir py-scrape-and-download
cd py-scrape-and-download/ |
The commands should work just as easily in Mac as in Linux. If you’re working in Windows, you’ll be on the “C:/” file structure, rather than the Unix-style structure above. So you might “mkdir” a new working directory in your “C:/TEMP” folder or wherever else you’d like to work. Or just make a folder wherever through Windows Explorer and “cd” there after the fact through the command line.
3. Create our python script in the text editor of your choice.
vim py-scrape-and-download.py |
The line above, which again should work for Linux or Mac, will open a new file in vim, the command-line text editor that I prefer. You can follow along, or feel free to make your own file in the application you prefer. If you’re a newbie Windows user, Notepad should work great.
If you’re following along in vim, you’ll need to enter “insert mode” so you can start entering text. Do that by hitting:
4. Write the code!
#!/usr/bin/env python
from mechanize import Browser
from BeautifulSoup import BeautifulSoup
mech = Browser()
url = "http://www.palewire.com/scrape/albums/2007.html"
page = mech.open(url)
html = page.read()
soup = BeautifulSoup(html)
print soup.prettify() |
Our first snippet of code, seen above, shows a basic introduction to each of our new modules.
After they’ve been imported in lines two and three, we put mechanize’s browser to use right away, storing it a variable I’ve decided to call mech, but which you could call anything you wanted (ex. browser, br, ie, whatever). We then use its open() method to grab the location of our first scrape target, my favorite albums of 2007, and store that in another variable we’ll call page.
That’s enough to go out on the web and grab the page, now we need to tell Python what to do with it. Mechanize’s read() method will return all of the HTML in the page, which we store, simply, in an variable called html and then pass to BeautifulSoup’s default method so it can be prepared for processing.
The reason we need to pass the page to Beautiful Soup is that there is a ton of HTML code in the page we don’t want. Our ultimate goal isn’t to print out the complete page source. We don’t want all the junky td and img and body tags. We want to free the data from the HTML by printing it out in a machine readable format we can repurpose for our own needs. In the next step we’ll ask Beautiful Soup to step through the code and pull out only the good parts, but here in the first iteration we’ll pause with just printing out the complete page code using a fun Beautiful Soup method called prettify(). It will spit out the HTML in a well-formed format. To take a look, save and quit out of your script (ESC, SHIFT+ZZ in vim) and fire it up from the command-line…
python py-scrape-and-download.py |
And you should see something like….
<html>
<head>
<title>
According to Ben...
</title>
</head>
<body>
<h2>
The 10 Greatest Albums in the History of 2007
</h2>
<table padding="1" width="60%" border="1" style="text-align:center;">
<tr style="font-weight:bold">
<td>
Rank
</td>
<td>
Artist
</td>
... |
…which means that you’ve successfully retrieved and printed out our first target. Now let’s move on to scraping the data out from the HTML.
#!/usr/bin/env python
from mechanize import Browser
from BeautifulSoup import BeautifulSoup
mech = Browser()
url = "http://www.palewire.com/scrape/albums/2007.html"
page = mech.open(url)
html = page.read()
soup = BeautifulSoup(html)
table = soup.find("table", border=1)
for row in table.findAll('tr')[1:]:
col = row.findAll('td')
rank = col[0].string
artist = col[1].string
album = col[2].string
cover_link = col[3].img['src']
record = (rank, artist, album, cover_link)
print "|".join(record) |
The second version of our script, seen above, removes the prettify() command that concluded version one and replaces it with the Beautiful Soup code necessary to parse the rankings from the page.
When you’re scraping a real target out there on the wild Web, the mechanize part of the script is likely to remain pretty much the same, but the Beautiful Soup portion that pulls the data from the page is going to have change each time, tailored to work with however your target HTML is structured.
So your job as the scraper is to inspect your target table and figure out how you can get Beautiful Soup to hone in on the elements you want to harvest. I like to do this using the Firefox plugin Firebug, which allows you to right-click and, by choosing the “Inspect Element” option, have the browser pull up and highlight the HTML underlying any portion of the page. But all that’s really necessary is that you take a look at the page’s source code.
Since most HTML pages you’ll be targeting, including my sample site, will include more than one set of table tags, you often have to find something unique about the table you’re after. This is necessary so that Beautiful Soup knows how to zoom in on that section of the code you’re after and ignore all the flotsam around it.
If you look closely at this particular page, you’ll note that while both table tags have the same width value, an easy way to distinguish them is that they have different border values…
<table width="60%" border="1" style="text-align: center;" padding="1">
...
<table width="60%" border="0"> |
…and the one we want to harvest has a border value of one. That’s why the first Beautiful Soup command seen in the snippet above uses the find() method to capture the table with that characteristic.
table = soup.find("table", border=1) |
Once that’s been accomplished, the new table variable is immediately put to use in a loop that is designed to step through each row and pull out the data we want.
for row in table.findAll('tr')[1:]: |
It uses Beautiful Soup’s findAll() method to put all of the tr tags (which is the HTML equivalent of a row) into a list. The [1:] modifier at the end instructs the loop to skip the first item, which, from looking at the page, we can tell is an unneeded header line.
Then, after the loop is set up on the tr tags, we set up another list that will grab all of the td tags (the HTML equivalent of a column) from each row.
Now pulling out the data is simply a matter of figuring out which order we can expect the data to appear in each row and pulling the corresponding values from the list. Since we expect rank, artist, album and cover to appear in each row from left to right, the first element of the col variable (col[0]) can always be expected to be the rank and the last element (col[3]) can always be expected to be the cover. So we create a new set of values to retrieve each, with some Beautiful Soup specific objects tacked on the end to grab only the bits we want.
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