When you’re scraping web pages, the most common task you need to perform is to extract data from the HTML source. There are several libraries available to achieve this:
- BeautifulSoup is a very popular screen scraping library among Python programmers which constructs a Python object based on the structure of the HTML code and also deals with bad markup reasonable well, but it has one drawback: it’s slow.
- lxml is a XML parsing library (which also parses HTML) with a pythonic API based on ElementTree (which is not part of the Python standard library).
Scrapy comes with its own mechanism for extracting data. They’re called XPath selectors (or just “selectors”, for short) because they “select” certain parts of the HTML document specified by XPath expressions.
XPath is a language for selecting nodes in XML documents, which can be used to with HTML.
This page explains how selectors work and describes their API which is very small and simple, unlike the lxml API which is much bigger because the lxml library can be use for many other tasks, besides selecting markup documents.
For a complete reference of the selectors API see the XPath selector reference.
There are two types of selectors bundled with Scrapy. Those are:
Both share the same selector API, and are constructed with a Response object as its first parameter. This is the Response they’re gonna be “selecting”.
hxs = HtmlXPathSelector(response) # a HTML selector xxs = XmlXPathSelector(response) # a XML selector
Using selectors with XPaths¶
To explain how to use the selectors we’ll use the Scrapy shell (which provides interactive testing) and an example page located in Scrapy documentation server:
Here’s its HTML code:
<html> <head> <base href='http://example.com/' /> <title>Example website</title> </head> <body> <div id='images'> <a href='image1.html'>Name: My image 1 <br /><img src='image1_thumb.jpg' /></a> <a href='image2.html'>Name: My image 2 <br /><img src='image2_thumb.jpg' /></a> <a href='image3.html'>Name: My image 3 <br /><img src='image3_thumb.jpg' /></a> <a href='image4.html'>Name: My image 4 <br /><img src='image4_thumb.jpg' /></a> <a href='image5.html'>Name: My image 5 <br /><img src='image5_thumb.jpg' /></a> </div> </body> </html>
First, let’s open the shell:
scrapy-ctl.py shell http://doc.scrapy.org/_static/selectors-sample1.html
Then, after the shell loads, you’ll have some selectors already instanced and ready to use.
Since we’re dealing with HTML we’ll be using the
HtmlXPathSelector object which is found, by default, in
hxs shell variable.
So, by looking at the HTML code of that page let’s construct an XPath (using an HTML selector) for selecting the text inside the title tag:
>>> hxs.select('//title/text()') [<HtmlXPathSelector (text) xpath=//title/text()>]
As you can see, the select() method returns a XPathSelectorList, which is a list of new selectors. This API can be used quickly for extracting nested data.
To actually extract the textual data you must call the selector
method, as follows:
>>> hxs.select('//title/text()').extract() [u'Example website']
Now we’re going to get the base URL and some image links:
>>> hxs.select('//base/@href').extract() [u'http://example.com/'] >>> hxs.select('//a[contains(@href, "image")]/@href').extract() [u'image1.html', u'image2.html', u'image3.html', u'image4.html', u'image5.html'] >>> hxs.select('//a[contains(@href, "image")]/img/@src').extract() [u'image1_thumb.jpg', u'image2_thumb.jpg', u'image3_thumb.jpg', u'image4_thumb.jpg', u'image5_thumb.jpg']
Using selectors with regular expressions¶
Selectors also have a
re() method for extracting data using regular
expressions. However, unlike using the
select() method, the
does not return a list of
XPathSelector objects, so you
can’t construct nested
Here’s an example used to extract images names from the HTML code above:
>>> hxs.select('//a[contains(@href, "image")]/text()').re(r'Name:\s*(.*)') [u'My image 1', u'My image 2', u'My image 3', u'My image 4', u'My image 5']
select() selector method returns a list of selectors, so you can call the
select() for those selectors too. Here’s an example:
>>> links = hxs.select('//a[contains(@href, "image")]') >>> links.extract() [u'<a href="image1.html">Name: My image 1 <br><img src="image1_thumb.jpg"></a>', u'<a href="image2.html">Name: My image 2 <br><img src="image2_thumb.jpg"></a>', u'<a href="image3.html">Name: My image 3 <br><img src="image3_thumb.jpg"></a>', u'<a href="image4.html">Name: My image 4 <br><img src="image4_thumb.jpg"></a>', u'<a href="image5.html">Name: My image 5 <br><img src="image5_thumb.jpg"></a>'] >>> for index, link in enumerate(links): args = (index, link.select('@href').extract(), link.select('img/@src').extract()) print 'Link number %d points to url %s and image %s' % args Link number 0 points to url [u'image1.html'] and image [u'image1_thumb.jpg'] Link number 1 points to url [u'image2.html'] and image [u'image2_thumb.jpg'] Link number 2 points to url [u'image3.html'] and image [u'image3_thumb.jpg'] Link number 3 points to url [u'image4.html'] and image [u'image4_thumb.jpg'] Link number 4 points to url [u'image5.html'] and image [u'image5_thumb.jpg']
Working with relative XPaths¶
Keep in mind that if you are nesting XPathSelectors and use an XPath that
/, that XPath will be absolute to the document and not relative
XPathSelector you’re calling it from.
For example, suppose you want to extract all
<p> elements inside
elements. First you get would get all
>>> divs = hxs.select('//div')
At first, you may be tempted to use the following approach, which is wrong, as
it actually extracts all
<p> elements from the document, not only those
>>> for p in divs.select('//p') # this is wrong - gets all <p> from the whole document >>> print p.extract()
This is the proper way to do it (note the dot prefixing the
>>> for p in divs.select('.//p') # extracts all <p> inside >>> print p.extract()
Another common case would be to extract all direct
>>> for p in divs.select('p') >>> print p.extract()
For more details about relative XPaths see the Location Paths section in the XPath specification.
Built-in XPath Selectors reference¶
There are two types of selectors bundled with Scrapy:
XmlXPathSelector. Both of them
implement the same
XPathSelector interface. The only different is that
one is used to process HTML data and the other XML data.
XPathSelectorobject is a wrapper over response to select certain parts of its content.
Responseobject that will be used for selecting and extracting data
xpathis a string containing the XPath to apply
Apply the given regex and return a list of unicode strings with the matches.
regexcan be either a compiled regular expression or a string which will be compiled to a regular expression using
Return a unicode string with the content of this
XPathSelectorwithout entities or CDATA. This method is intended to be use for text-only selectors, like
//h1). If it’s used for
XPathSelectorobjects which don’t select a textual content (ie. if they contain tags), the output of this method is undefined.
Register the given namespace to be used in this
XPathSelector. Without registering namespaces you can’t select or extract data from non-standard namespaces. See examples below.
XPathSelectorListclass is subclass of the builtin
listclass, which provides a few additional methods.
xpathis the same argument as the one in
regexis the same argument as the one in
XPathSelector.extract_unoquoted()method for all
XPathSelectorobjects in this list and return their results flattened, as a list of unicode strings. This method should not be applied to all kinds of XPathSelectors. For more info see
x = HtmlXPathSelector(html_response)
Extract the text of all
<h1>elements from a HTML response body, returning a list of unicode strings:
x.select("//h1").extract() # this includes the h1 tag x.select("//h1/text()").extract() # this excludes the h1 tag
Iterate over all
<p>tags and print their class attribute:
for node in x.select("//p"): ... print node.select("@href")
Extract textual data from all
<p>tags without entities, as a list of unicode strings:
x.select("//p/text()").extract_unquoted() # the following line is wrong. extract_unquoted() should only be used # with textual XPathSelectors x.select("//p").extract_unquoted() # it may work but output is unpredictable
x = HtmlXPathSelector(xml_response)
Extract all prices from a Google Base XML feed which requires registering a namespace:
x.register_namespace("g", "http://base.google.com/ns/1.0") x.select("//g:price").extract()