Last time we implemented a basic web scraper that downloaded the latest questions from StackOverflow and stored the results in MongoDB. In this article we’ll extend our scraper so that it crawls through the pagination links at the bottom of each page and scrapes the questions (question title and URL) from each page.
Free Bonus: Click here to download a Python + MongoDB project skeleton with full source code that shows you how to access MongoDB from Python.
Updates:
Before you start any scraping job, review the site’s terms of use policy and respect the robots.txt file. Also, adhere to ethical scraping practices by not flooding a site with numerous requests over a short span of time. Treat any site you scrape as if it were your own.
This is a collaboration piece between the folks at Real Python and György - a Python enthusiast and software developer, currently working at a big data company and seeking a new job at the same time. You can ask him questions on twitter - @kissgyorgy.
Getting Started
There are two possible ways to continue from where we left off.
The first is to extend our existing Spider by extracting every next page link from the response in the parse_item method with an xpath expression and just yield a Request object with a callback to the same parse_item method. This way scrapy will automatically make a new request to the link we specify. You can find more information on this method in the Scrapy documentation.
The other, much simpler option is to utilize a different type of spider - the CrawlSpider (link). It’s an extended version of the basic Spider, designed exactly for our use case.
The CrawlSpider
We’ll be using the same Scrapy project from the last tutorial, so grab the code from the repo if you need it.
Create the Boilerplate
Within the “stack” directory, start by generating the spider boilerplate from the crawl template:
$ scrapy genspider stack_crawler stackoverflow.com -t crawl
Created spider 'stack_crawler' using template 'crawl' in module:
stack.spiders.stack_crawler
The Scrapy project should now look like this:
├── scrapy.cfg
└── stack
├── __init__.py
├── items.py
├── pipelines.py
├── settings.py
└── spiders
├── __init__.py
├── stack_crawler.py
└── stack_spider.py
And the stack_crawler.py file should look like this:
# -*- coding: utf-8 -*-
import scrapy
from scrapy.contrib.linkextractors import LinkExtractor
from scrapy.contrib.spiders import CrawlSpider, Rule
from stack.items import StackItem
class StackCrawlerSpider(CrawlSpider):
name = 'stack_crawler'
allowed_domains = ['stackoverflow.com']
start_urls = ['http://www.stackoverflow.com/']
rules = (
Rule(LinkExtractor(allow=r'Items/'), callback='parse_item', follow=True),
)
def parse_item(self, response):
i = StackItem()
#i['domain_id'] = response.xpath('//input[@id="sid"]/@value').extract()
#i['name'] = response.xpath('//div[@id="name"]').extract()
#i['description'] = response.xpath('//div[@id="description"]').extract()
return i
We just need to make a few updates to this boilerplate…
Update the start_urls list
First, add the first page of questions to the start_urls list:
start_urls = [
'http://stackoverflow.com/questions?pagesize=50&sort=newest'
]
Update the rules list
Next, we need to tell the spider where it can find the next page links by adding a regular expression to the rules attribute:
rules = [
Rule(LinkExtractor(allow=r'questions\?page=[0-9]&sort=newest'),
callback='parse_item', follow=True)
]
Scrapy will now automatically request new pages based on those links and pass the response to the parse_item method to extract the questions and titles.
If you’re paying close attention, this regex limits the crawling to the first 9 pages since for this demo we do not want to scrape all 176,234 pages!
Update the parse_item method
Now we just need to write how to parse the pages with xpath, which we already did in the last tutorial - so just copy it over:
def parse_item(self, response):
questions = response.xpath('//div[@class="summary"]/h3')
for question in questions:
item = StackItem()
item['url'] = question.xpath(
'a[@class="question-hyperlink"]/@href').extract()[0]
item['title'] = question.xpath(
'a[@class="question-hyperlink"]/text()').extract()[0]
yield item
That’s it for the spider, but do not start it just yet.
Add a Download Delay
We need to be nice to StackOverflow (and any site, for that matter) by setting a download delay in settings.py:
DOWNLOAD_DELAY = 5
This tells Scrapy to wait at least 5 seconds between every new request it makes. You’re essentially rate limiting yourself. If you do not do this, StackOverflow will rate limit you; and if you continue to scrape the site without imposing a rate limit, your IP address could be banned. So, be nice - Treat any site you scrape as if it were your own.
Now there is only one thing left to do - store the data.
MongoDB
Last time we only downloaded 50 questions, but since we are grabbing a lot more data this time, we want to avoid adding duplicate questions to the database. We can do that by using a MongoDB upsert, which means we update the question title if it is already in the database and insert otherwise.
Modify the MongoDBPipeline we defined earlier:
class MongoDBPipeline(object):
def __init__(self):
connection = pymongo.MongoClient(
settings['MONGODB_SERVER'],
settings['MONGODB_PORT']
)
db = connection[settings['MONGODB_DB']]
self.collection = db[settings['MONGODB_COLLECTION']]
def process_item(self, item, spider):
for data in item:
if not data:
raise DropItem("Missing data!")
self.collection.update({'url': item['url']}, dict(item), upsert=True)
log.msg("Question added to MongoDB database!",
level=log.DEBUG, spider=spider)
return item
For simplicity, we did not optimize the query and did not deal with indexes since this is not a production environment.
Test
Start the spider!
$ scrapy crawl stack_crawler
Now sit back and watch your database fill with data!
$ mongo
MongoDB shell version: 3.0.4
> use stackoverflow
switched to db stackoverflow
> db.questions.count()
447
>
Conclusion
You can download the entire source code from the Github repository. Comment below with questions. Cheers!
Free Bonus: Click here to download a Python + MongoDB project skeleton with full source code that shows you how to access MongoDB from Python.
Looking for more web scraping? Be sure to check out the Real Python courses. Looking to hire a professional web scraper? Check out GoScrape.
