Usage

Getting started

If you already know how to write CSS or XPath expressions, using Parsel is straightforward: you just need to create a Selector object for the HTML or XML text you want to parse, and use the available methods for selecting parts from the text and extracting data out of the result.

Creating a Selector object is simple:

>>> from parsel import Selector
>>> text = u"<html><body><h1>Hello, Parsel!</h1></body></html>"
>>> sel = Selector(text=text)

Note

One important thing to note is that if you’re using Python 2, make sure to use an unicode object for the text argument. Selector expects text to be an unicode object in Python 2 or an str object in Python 3.

Once you have created the Selector object, you can use CSS or XPath expressions to select elements:

>>> sel.css('h1')
[<Selector xpath=u'descendant-or-self::h1' data=u'<h1>Hello, Parsel!</h1>'>]
>>> sel.xpath('//h1')  # the same, but now with XPath
[<Selector xpath='//h1' data=u'<h1>Hello, Parsel!</h1>'>]

And extract data from those elements:

>>> sel.xpath('//h1/text()').extract()
[u'Hello, Parsel!']
>>> sel.css('h1::text').extract_first()
u'Hello, Parsel!'

XPath is a language for selecting nodes in XML documents, which can also be used with HTML. CSS is a language for applying styles to HTML documents. It defines selectors to associate those styles with specific HTML elements.

You can use either language you’re more comfortable with, though you may find that in some specific cases XPath is more powerful than CSS.

Using selectors

To explain how to use the selectors we’ll use the requests library to download an example page located in the Parsel’s documentation:

For the sake of completeness, here’s its full 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>

So, let’s download that page and create a selector for it:

>>> import requests
>>> from parsel import Selector
>>> url = 'http://parsel.readthedocs.org/en/latest/_static/selectors-sample1.html'
>>> text = requests.get(url).text
>>> selector = Selector(text=text)

Since we’re dealing with HTML, the default type for Selector, we don’t need to specify the type argument.

So, by looking at the HTML code of that page, let’s construct an XPath for selecting the text inside the title tag:

>>> selector.xpath('//title/text()')
[<Selector xpath='//title/text()' data=u'Example website'>]

You can also ask the same thing using CSS instead:

>>> selector.css('title::text')
[<Selector xpath=u'descendant-or-self::title/text()' data=u'Example website'>]

As you can see, .xpath() and .css() methods return a SelectorList instance, which is a list of new selectors. This API can be used for quickly selecting nested data:

>>> selector.css('img').xpath('@src').extract()
[u'image1_thumb.jpg',
 u'image2_thumb.jpg',
 u'image3_thumb.jpg',
 u'image4_thumb.jpg',
 u'image5_thumb.jpg']

To actually extract the textual data, you must call the selector .extract() method, as follows:

>>> selector.xpath('//title/text()').extract()
[u'Example website']

If you want to extract only first matched element, you can call the selector .extract_first():

>>> selector.xpath('//div[@id="images"]/a/text()').extract_first()
u'Name: My image 1 '

It returns None if no element was found:

>>> selector.xpath('//div[@id="not-exists"]/text()').extract_first() is None
True

Notice that CSS selectors can select text or attribute nodes using CSS3 pseudo-elements:

>>> selector.css('title::text').extract()
[u'Example website']

Now we’re going to get the base URL and some image links:

>>> selector.xpath('//base/@href').extract()
[u'http://example.com/']

>>> selector.css('base::attr(href)').extract()
[u'http://example.com/']

>>> selector.xpath('//a[contains(@href, "image")]/@href').extract()
[u'image1.html',
 u'image2.html',
 u'image3.html',
 u'image4.html',
 u'image5.html']

>>> selector.css('a[href*=image]::attr(href)').extract()
[u'image1.html',
 u'image2.html',
 u'image3.html',
 u'image4.html',
 u'image5.html']

>>> selector.xpath('//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']

>>> selector.css('a[href*=image] img::attr(src)').extract()
[u'image1_thumb.jpg',
 u'image2_thumb.jpg',
 u'image3_thumb.jpg',
 u'image4_thumb.jpg',
 u'image5_thumb.jpg']

Extensions to CSS Selectors

Per W3C standards, CSS selectors do not support selecting text nodes or attribute values. But selecting these is so essential in a web scraping context that Parsel implements a couple of non-standard pseudo-elements:

  • to select text nodes, use ::text
  • to select attribute values, use ::attr(name) where name is the name of the attribute that you want the value of

Warning

These pseudo-elements are Scrapy-/Parsel-specific. They will most probably not work with other libraries like lxml or PyQuery.

Examples:

  • title::text selects children text nodes of a descendant <title> element:

    >>> selector.css('title::text').extract_first()
    u'Example website'
    
  • *::text selects all descendant text nodes of the current selector context:

    >>> selector.css('#images *::text').extract()
    [u'\n   ',
     u'Name: My image 1 ',
     u'\n   ',
     u'Name: My image 2 ',
     u'\n   ',
     u'Name: My image 3 ',
     u'\n   ',
     u'Name: My image 4 ',
     u'\n   ',
     u'Name: My image 5 ',
     u'\n  ']
    
  • a::attr(href) selects the href attribute value of descendant links:

    >>> selector.css('a::attr(href)').extract()
    [u'image1.html',
     u'image2.html',
     u'image3.html',
     u'image4.html',
     u'image5.html']
    

Note

You cannot chain these pseudo-elements. But in practice it would not make much sense: text nodes do not have attributes, and attribute values are string values already and do not have children nodes.

Nesting selectors

The selection methods (.xpath() or .css()) return a list of selectors of the same type, so you can call the selection methods for those selectors too. Here’s an example:

>>> links = selector.xpath('//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.xpath('@href').extract(), link.xpath('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']

Using selectors with regular expressions

Selector also has a .re() method for extracting data using regular expressions. However, unlike using .xpath() or .css() methods, .re() returns a list of unicode strings. So you can’t construct nested .re() calls.

Here’s an example used to extract image names from the HTML code above:

>>> selector.xpath('//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']

There’s an additional helper reciprocating .extract_first() for .re(), named .re_first(). Use it to extract just the first matching string:

>>> selector.xpath('//a[contains(@href, "image")]/text()').re_first(r'Name:\s*(.*)')
u'My image 1'

Working with relative XPaths

Keep in mind that if you are nesting selectors and use an XPath that starts with /, that XPath will be absolute to the document and not relative to the Selector you’re calling it from.

For example, suppose you want to extract all <p> elements inside <div> elements. First, you would get all <div> elements:

>>> divs = selector.xpath('//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 inside <div> elements:

>>> for p in divs.xpath('//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 .//p XPath):

>>> for p in divs.xpath('.//p'):  # extracts all <p> inside
...     print p.extract()

Another common case would be to extract all direct <p> children:

>>> for p in divs.xpath('p'):
...     print p.extract()

For more details about relative XPaths see the Location Paths section in the XPath specification.

Using EXSLT extensions

Being built atop lxml, parsel selectors support some EXSLT extensions and come with these pre-registered namespaces to use in XPath expressions:

prefix namespace usage
re http://exslt.org/regular-expressions regular expressions
set http://exslt.org/sets set manipulation

Regular expressions

The test() function, for example, can prove quite useful when XPath’s starts-with() or contains() are not sufficient.

Example selecting links in list item with a “class” attribute ending with a digit:

>>> from parsel import Selector
>>> doc = u"""
... <div>
...     <ul>
...         <li class="item-0"><a href="link1.html">first item</a></li>
...         <li class="item-1"><a href="link2.html">second item</a></li>
...         <li class="item-inactive"><a href="link3.html">third item</a></li>
...         <li class="item-1"><a href="link4.html">fourth item</a></li>
...         <li class="item-0"><a href="link5.html">fifth item</a></li>
...     </ul>
... </div>
... """
>>> sel = Selector(text=doc)
>>> sel.xpath('//li//@href').extract()
[u'link1.html', u'link2.html', u'link3.html', u'link4.html', u'link5.html']
>>> sel.xpath('//li[re:test(@class, "item-\d$")]//@href').extract()
[u'link1.html', u'link2.html', u'link4.html', u'link5.html']
>>>

Warning

C library libxslt doesn’t natively support EXSLT regular expressions so lxml‘s implementation uses hooks to Python’s re module. Thus, using regexp functions in your XPath expressions may add a small performance penalty.

Set operations

These can be handy for excluding parts of a document tree before extracting text elements for example.

Example extracting microdata (sample content taken from http://schema.org/Product) with groups of itemscopes and corresponding itemprops:

>>> doc = u"""
... <div itemscope itemtype="http://schema.org/Product">
...   <span itemprop="name">Kenmore White 17" Microwave</span>
...   <img src="kenmore-microwave-17in.jpg" alt='Kenmore 17" Microwave' />
...   <div itemprop="aggregateRating"
...     itemscope itemtype="http://schema.org/AggregateRating">
...    Rated <span itemprop="ratingValue">3.5</span>/5
...    based on <span itemprop="reviewCount">11</span> customer reviews
...   </div>
...
...   <div itemprop="offers" itemscope itemtype="http://schema.org/Offer">
...     <span itemprop="price">$55.00</span>
...     <link itemprop="availability" href="http://schema.org/InStock" />In stock
...   </div>
...
...   Product description:
...   <span itemprop="description">0.7 cubic feet countertop microwave.
...   Has six preset cooking categories and convenience features like
...   Add-A-Minute and Child Lock.</span>
...
...   Customer reviews:
...
...   <div itemprop="review" itemscope itemtype="http://schema.org/Review">
...     <span itemprop="name">Not a happy camper</span> -
...     by <span itemprop="author">Ellie</span>,
...     <meta itemprop="datePublished" content="2011-04-01">April 1, 2011
...     <div itemprop="reviewRating" itemscope itemtype="http://schema.org/Rating">
...       <meta itemprop="worstRating" content = "1">
...       <span itemprop="ratingValue">1</span>/
...       <span itemprop="bestRating">5</span>stars
...     </div>
...     <span itemprop="description">The lamp burned out and now I have to replace
...     it. </span>
...   </div>
...
...   <div itemprop="review" itemscope itemtype="http://schema.org/Review">
...     <span itemprop="name">Value purchase</span> -
...     by <span itemprop="author">Lucas</span>,
...     <meta itemprop="datePublished" content="2011-03-25">March 25, 2011
...     <div itemprop="reviewRating" itemscope itemtype="http://schema.org/Rating">
...       <meta itemprop="worstRating" content = "1"/>
...       <span itemprop="ratingValue">4</span>/
...       <span itemprop="bestRating">5</span>stars
...     </div>
...     <span itemprop="description">Great microwave for the price. It is small and
...     fits in my apartment.</span>
...   </div>
...   ...
... </div>
... """
>>> sel = Selector(text=doc, type="html")
>>> for scope in sel.xpath('//div[@itemscope]'):
...     print "current scope:", scope.xpath('@itemtype').extract()
...     props = scope.xpath('''
...                 set:difference(./descendant::*/@itemprop,
...                                .//*[@itemscope]/*/@itemprop)''')
...     print "    properties:", props.extract()
...     print

current scope: [u'http://schema.org/Product']
    properties: [u'name', u'aggregateRating', u'offers', u'description', u'review', u'review']

current scope: [u'http://schema.org/AggregateRating']
    properties: [u'ratingValue', u'reviewCount']

current scope: [u'http://schema.org/Offer']
    properties: [u'price', u'availability']

current scope: [u'http://schema.org/Review']
    properties: [u'name', u'author', u'datePublished', u'reviewRating', u'description']

current scope: [u'http://schema.org/Rating']
    properties: [u'worstRating', u'ratingValue', u'bestRating']

current scope: [u'http://schema.org/Review']
    properties: [u'name', u'author', u'datePublished', u'reviewRating', u'description']

current scope: [u'http://schema.org/Rating']
    properties: [u'worstRating', u'ratingValue', u'bestRating']

>>>

Here we first iterate over itemscope elements, and for each one, we look for all itemprops elements and exclude those that are themselves inside another itemscope.

Some XPath tips

Here are some tips that you may find useful when using XPath with Parsel, based on this post from ScrapingHub’s blog. If you are not much familiar with XPath yet, you may want to take a look first at this XPath tutorial.

Using text nodes in a condition

When you need to use the text content as argument to an XPath string function, avoid using .//text() and use just . instead.

This is because the expression .//text() yields a collection of text elements – a node-set. And when a node-set is converted to a string, which happens when it is passed as argument to a string function like contains() or starts-with(), it results in the text for the first element only.

Example:

>>> from parsel import Selector
>>> sel = Selector(text='<a href="#">Click here to go to the <strong>Next Page</strong></a>')

Converting a node-set to string:

>>> sel.xpath('//a//text()').extract() # take a peek at the node-set
[u'Click here to go to the ', u'Next Page']
>>> sel.xpath("string(//a[1]//text())").extract() # convert it to string
[u'Click here to go to the ']

A node converted to a string, however, puts together the text of itself plus of all its descendants:

>>> sel.xpath("//a[1]").extract() # select the first node
[u'<a href="#">Click here to go to the <strong>Next Page</strong></a>']
>>> sel.xpath("string(//a[1])").extract() # convert it to string
[u'Click here to go to the Next Page']

So, using the .//text() node-set won’t select anything in this case:

>>> sel.xpath("//a[contains(.//text(), 'Next Page')]").extract()
[]

But using the . to mean the node, works:

>>> sel.xpath("//a[contains(., 'Next Page')]").extract()
[u'<a href="#">Click here to go to the <strong>Next Page</strong></a>']

Beware of the difference between //node[1] and (//node)[1]

//node[1] selects all the nodes occurring first under their respective parents.

(//node)[1] selects all the nodes in the document, and then gets only the first of them.

Example:

>>> from parsel import Selector
>>> sel = Selector(text="""
....:     <ul class="list">
....:         <li>1</li>
....:         <li>2</li>
....:         <li>3</li>
....:     </ul>
....:     <ul class="list">
....:         <li>4</li>
....:         <li>5</li>
....:         <li>6</li>
....:     </ul>""")
>>> xp = lambda x: sel.xpath(x).extract()

This gets all first <li> elements under whatever it is its parent:

>>> xp("//li[1]")
[u'<li>1</li>', u'<li>4</li>']

And this gets the first <li> element in the whole document:

>>> xp("(//li)[1]")
[u'<li>1</li>']

This gets all first <li> elements under an <ul> parent:

>>> xp("//ul/li[1]")
[u'<li>1</li>', u'<li>4</li>']

And this gets the first <li> element under an <ul> parent in the whole document:

>>> xp("(//ul/li)[1]")
[u'<li>1</li>']

When querying by class, consider using CSS

Because an element can contain multiple CSS classes, the XPath way to select elements by class is the rather verbose:

*[contains(concat(' ', normalize-space(@class), ' '), ' someclass ')]

If you use @class='someclass' you may end up missing elements that have other classes, and if you just use contains(@class, 'someclass') to make up for that you may end up with more elements that you want, if they have a different class name that shares the string someclass.

As it turns out, parsel selectors allow you to chain selectors, so most of the time you can just select by class using CSS and then switch to XPath when needed:

>>> from parsel import Selector
>>> sel = Selector(text='<div class="hero shout"><time datetime="2014-07-23 19:00">Special date</time></div>')
>>> sel.css('.shout').xpath('./time/@datetime').extract()
[u'2014-07-23 19:00']

This is cleaner than using the verbose XPath trick shown above. Just remember to use the . in the XPath expressions that will follow.

API reference

Selector objects

class parsel.selector.Selector(text=None, type=None, namespaces=None, root=None, base_url=None, _expr=None)[source]

Selector allows you to select parts of an XML or HTML text using CSS or XPath expressions and extract data from it.

text is a unicode object in Python 2 or a str object in Python 3

type defines the selector type, it can be "html", "xml" or None (default). If type is None, the selector defaults to "html".

css(query)[source]

Apply the given CSS selector and return a SelectorList instance.

query is a string containing the CSS selector to apply.

In the background, CSS queries are translated into XPath queries using cssselect library and run .xpath() method.

extract()[source]

Serialize and return the matched nodes in a single unicode string. Percent encoded content is unquoted.

re(regex)[source]

Apply the given regex and return a list of unicode strings with the matches.

regex can be either a compiled regular expression or a string which will be compiled to a regular expression using re.compile(regex)

register_namespace(prefix, uri)[source]

Register the given namespace to be used in this Selector. Without registering namespaces you can’t select or extract data from non-standard namespaces. See Working on XML (and namespaces).

remove_namespaces()[source]

Remove all namespaces, allowing to traverse the document using namespace-less xpaths. See Removing namespaces.

selectorlist_cls

alias of SelectorList

xpath(query, namespaces=None, **kwargs)[source]

Find nodes matching the xpath query and return the result as a SelectorList instance with all elements flattened. List elements implement Selector interface too.

query is a string containing the XPATH query to apply.

namespaces is an optional prefix: namespace-uri mapping (dict) for additional prefixes to those registered with register_namespace(prefix, uri). Contrary to register_namespace(), these prefixes are not saved for future calls.

Any additional named arguments can be used to pass values for XPath variables in the XPath expression, e.g.:

selector.xpath('//a[href=$url]', url="http://www.example.com")

SelectorList objects

class parsel.selector.SelectorList[source]

The SelectorList class is a subclass of the builtin list class, which provides a few additional methods.

css(query)[source]

Call the .css() method for each element in this list and return their results flattened as another SelectorList.

query is the same argument as the one in Selector.css()

extract()[source]

Call the .extract() method for each element is this list and return their results flattened, as a list of unicode strings.

extract_first(default=None)[source]

Return the result of .extract() for the first element in this list. If the list is empty, return the default value.

re(regex)[source]

Call the .re() method for each element in this list and return their results flattened, as a list of unicode strings.

re_first(regex, default=None)[source]

Call the .re() method for the first element in this list and return the result in an unicode string. If the list is empty or the regex doesn’t match anything, return the default value (None if the argument is not provided).

xpath(xpath, namespaces=None, **kwargs)[source]

Call the .xpath() method for each element in this list and return their results flattened as another SelectorList.

query is the same argument as the one in Selector.xpath()

namespaces is an optional prefix: namespace-uri mapping (dict) for additional prefixes to those registered with register_namespace(prefix, uri). Contrary to register_namespace(), these prefixes are not saved for future calls.

Any additional named arguments can be used to pass values for XPath variables in the XPath expression, e.g.:

selector.xpath('//a[href=$url]', url="http://www.example.com")

Working on HTML

Here are some Selector examples to illustrate several concepts. In all cases, we assume there is already a Selector instantiated with an HTML text like this:

sel = Selector(text=html_text)
  1. Select all <h1> elements from an HTML text, returning a list of Selector objects (ie. a SelectorList object):

    sel.xpath("//h1")
    
  2. Extract the text of all <h1> elements from an HTML text, returning a list of unicode strings:

    sel.xpath("//h1").extract()         # this includes the h1 tag
    sel.xpath("//h1/text()").extract()  # this excludes the h1 tag
    
  3. Iterate over all <p> tags and print their class attribute:

    for node in sel.xpath("//p"):
        print node.xpath("@class").extract()
    

Working on XML (and namespaces)

Here are some examples to illustrate concepts for Selector objects instantiated with an XML text like this:

sel = Selector(text=xml_text, type='xml')
  1. Select all <product> elements from an XML text, returning a list of Selector objects (ie. a SelectorList object):

    sel.xpath("//product")
    
  2. Extract all prices from a Google Base XML feed which requires registering a namespace:

    sel.register_namespace("g", "http://base.google.com/ns/1.0")
    sel.xpath("//g:price").extract()
    

Removing namespaces

When dealing with scraping projects, it is often quite convenient to get rid of namespaces altogether and just work with element names, to write more simple/convenient XPaths. You can use the Selector.remove_namespaces() method for that.

Let’s show an example that illustrates this with Github blog atom feed.

Let’s download the atom feed using requests and create a selector:

>>> import requests
>>> from parsel import Selector
>>> text = requests.get('https://github.com/blog.atom').text
>>> sel = Selector(text=text, type='xml')

This is how the file starts:

<?xml version="1.0" encoding="UTF-8"?>
<feed xml:lang="en-US"
      xmlns="http://www.w3.org/2005/Atom"
      xmlns:media="http://search.yahoo.com/mrss/">
  <id>tag:github.com,2008:/blog</id>
  ...

You can see two namespace declarations: a default “http://www.w3.org/2005/Atom” and another one using the “media:” prefix for “http://search.yahoo.com/mrss/”.

We can try selecting all <link> objects and then see that it doesn’t work (because the Atom XML namespace is obfuscating those nodes):

>>> sel.xpath("//link")
[]

But once we call the Selector.remove_namespaces() method, all nodes can be accessed directly by their names:

>>> sel.remove_namespaces()
>>> sel.xpath("//link")
[<Selector xpath='//link' data=u'<link xmlns="http://www.w3.org/2005/Atom'>,
 <Selector xpath='//link' data=u'<link xmlns="http://www.w3.org/2005/Atom'>,
 ...

If you wonder why the namespace removal procedure isn’t called always by default instead of having to call it manually, this is because of two reasons, which, in order of relevance, are:

  1. Removing namespaces requires to iterate and modify all nodes in the document, which is a reasonably expensive operation to perform by default for all documents.
  2. There could be some cases where using namespaces is actually required, in case some element names clash between namespaces. These cases are very rare though.

Ad-hoc namespaces references

Selector objects also allow passing namespaces references along with the query, through a namespaces argument, with the prefixes you declare being used in your XPath or CSS query.

Let’s use the same Atom feed from Github:

>>> import requests
>>> from parsel import Selector
>>> text = requests.get('https://github.com/blog.atom').text
>>> sel = Selector(text=text, type='xml')

And try to select the links again, now using an “atom:” prefix for the “link” node test:

>>> sel.xpath("//atom:link", namespaces={"atom": "http://www.w3.org/2005/Atom"})
[<Selector xpath='//atom:link' data='<link xmlns="http://www.w3.org/2005/Atom'>,
 <Selector xpath='//atom:link' data='<link xmlns="http://www.w3.org/2005/Atom'>,
 ...

You can pass several namespaces (here we’re using shorter 1-letter prefixes):

>>> sel.xpath("//a:entry/m:thumbnail/@url",
...               namespaces={"a": "http://www.w3.org/2005/Atom",
...                           "m": "http://search.yahoo.com/mrss/"}).extract()
['https://avatars1.githubusercontent.com/u/11529908?v=3&s=60',
 'https://avatars0.githubusercontent.com/u/15114852?v=3&s=60',
 ...

Variables in XPath expressions

XPath allows you to reference variables in your XPath expressions, using the $somevariable syntax. This is somewhat similar to parameterized queries or prepared statements in the SQL world where you replace some arguments in your queries with placeholders like ?, which are then substituted with values passed with the query.

Here’s an example to match an element based on its normalized string-value:

>>> str_to_match = "Name: My image 3"
>>> selector.xpath('//a[normalize-space(.)=$match]',
...                match=str_to_match).extract_first()
u'<a href="image3.html">Name: My image 3 <br><img src="image3_thumb.jpg"></a>'

All variable references must have a binding value when calling .xpath() (otherwise you’ll get a ValueError: XPath error: exception). This is done by passing as many named arguments as necessary.

Here’s another example using a position range passed as two integers:

>>> start, stop = 2, 4
>>> selector.xpath('//a[position()>=$_from and position()<=$_to]',
...                _from=start, _to=stop).extract()
[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>']

Named variables can be useful when strings need to be escaped for single or double quotes characters. The example below would be a bit tricky to get right (or legible) without a variable reference:

>>> html = u'''<html>
... <body>
...   <p>He said: "I don't know why, but I like mixing single and double quotes!"</p>
... </body>
... </html>'''
>>> selector = Selector(text=html)
>>>
>>> selector.xpath('//p[contains(., $mystring)]',
...                mystring='''He said: "I don't know''').extract_first()
u'<p>He said: "I don\'t know why, but I like mixing single and double quotes!"</p>'

Converting CSS to XPath

parsel.css2xpath(query)[source]

Return translated XPath version of a given CSS query

When you’re using an API that only accepts XPath expressions, it’s sometimes useful to convert CSS to XPath. This allows you to take advantage of the conciseness of CSS to query elements by classes and the easeness of manipulating XPath expressions at the same time.

On those occasions, use the function css2xpath():

>>> from parsel import css2xpath
>>> css2xpath('h1.title')
u"descendant-or-self::h1[@class and contains(concat(' ', normalize-space(@class), ' '), ' title ')]"
>>> css2xpath('.profile-data') + '//h2'
u"descendant-or-self::*[@class and contains(concat(' ', normalize-space(@class), ' '), ' profile-data ')]//h2"

As you can see from the examples above, it returns the translated CSS query into an XPath expression as a string, which you can use as-is or combine to build a more complex expression, before feeding to a function expecting XPath.

Similar libraries

  • 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 reasonably well.
  • lxml is an XML parsing library (which also parses HTML) with a pythonic API based on ElementTree. (lxml is not part of the Python standard library.). Parsel uses it under-the-hood.
  • PyQuery is a library that, like Parsel, uses lxml and cssselect under the hood, but it offers a jQuery-like API to traverse and manipulate XML/HTML documents.

Parsel is built on top of the lxml library, which means they’re very similar in speed and parsing accuracy. The advantage of using Parsel over lxml is that Parsel is simpler to use and extend, unlike the lxml API which is much bigger because the lxml library can be used for many other tasks, besides selecting markup documents.