Selecting content on a web page with XPath

Overview

Teaching: 30 min
Exercises: 15 min
Questions
  • How can I select a specific element on web page?

  • What is XPath and how can I use it?

Objectives
  • Introduce XPath queries

  • Explain the structure of an XML or HTML document

  • Explain how to view the underlying HTML content of a web page in a browser

  • Explain how to run XPath queries in a browser

  • Introduce the XPath syntax

  • Use the XPath syntax to select elements on this web page

Before we delve into web scraping proper, we will first spend some time introducing some of the techniques that are required to indicate exactly what should be extracted from the web pages we aim to scrape.

The material in this section was adapted from the XPath and XQuery Tutorial written by Kim Pham (@tolloid) for the July 2016 Library Carpentry workshop in Toronto.

Introduction

XPath (which stands for XML Path Language) is an expression language used to specify parts of an XML document. XPath is rarely used on its own, rather it is used within software and languages that are aimed at manipulating XML documents, such as XSLT, XQuery or the web scraping tools that will be introduced later in this lesson. XPath can also be used in documents with a structure that is similar to XML, like HTML.

Markup Languages

XML and HTML are markup languages. This means that they use a set of tags or rules to organise and provide information about the data they contain. This structure helps to automate processing, editing, formatting, displaying, printing, etc. that information.

XML documents stores data in plain text format. This provides a software- and hardware-independent way of storing, transporting, and sharing data. XML format is an open format, meant to be software agnostic. You can open an XML document in any text editor and the data it contains will be shown as it is meant to be represented. This allows for exchange between incompatible systems and easier conversion of data.

XML and HTML

Note that HTML and XML have a very similar structure, which is why XPath can be used almost interchangeably to navigate both HTML and XML documents. In fact, starting with HTML5, HTML documents are fully-formed XML documents. In a sense, HTML is like a particular dialect of XML.

XML document follows basic syntax rules:

<catfood>
  <manufacturer>Purina</manufacturer>
    <address> 12 Cat Way, Boise, Idaho, 21341</address>
  <date>2019-10-01</date>
</catfood>

XPath Expressions

XPath is written using expressions, and when these expressions are evaluated on XML documents they return an object containing the node(s) that you aim to select. Contrary to a flat text document, XML data is structured, as it is organized in nodes and subnodes. Therefore, when using XPath, we are not querying raw text or data values like we would so using Regular Expressions, for example. Instead, XPath makes use of the fact that XML documents are structured and instead navigates through the node structure to select the data that we are looking for.

XPath is typically used to select and compare nodes, not edit them. To manipulate or edit nodes, another language such as XQuery would be used instead.

XPath assumes structured data

We can think of using XPath as similar to search a library catalogue using the advanced search function. In a catalogue, we can take advantage of the fact that bibliographic information has been properly structured in the database by specifying which metadata fields we want to query. For example, if we are looking for books about Shakespeare but not those written by him, we can use the advanced search function to look for that name in the “title” field only.

Contrary to Regular Expressions, this means that we don’t have to know in advance what the data we are looking for looks like, we just need to know in which node(s) (or fields) it resides.

Now let’s start using XPath.

A popular way to represent the structure of an XML or HTML document is the node tree:

HTML Node Tree

In an HTML document, everything is a node:

The nodes in such a tree have a hierarchical relationship to each other. We use the terms parent, child and sibling to describe these relationships:

Node relationships

Paths in XPath are defined using slashes (/) to separate the steps in a node connection sequence, much like URLs or Unix directories.

In XPath, all expressions are evaluated based on a context node. The context node is the node in which a path starts from. The default context is the root node, indicated by a single slash (/), as in the example above.

The most useful path expressions are listed below:

Expression Description
nodename Select all nodes with the name “nodename”
/ A beginning single slash indicates a select from the root node, subsequent slashes indicate selecting a child node from current node
// Select direct and indirect child nodes in the document from the current node - this gives us the ability to “skip levels”
. Select the current context node
.. Select the parent of the context node
@ Select attributes of the context node
[@attribute = 'value'] Select nodes with a particular attribute value
text() Select the text content of a node
| Pipe chains expressions and brings back results from either expression, think of a set union

We will use the HTML code that describes this very page you are reading as an example. By default, a web browser interprets the HTML code to determine what markup to apply to the various elements of a document, and the code is invisible. To make the underlying code visible, all browsers have a function to display the raw HTML content of a web page.

Display the source of this page

Using your favourite browser, display the HTML source code of this page.

Tip: in most browsers, all you have to do is do a right-click anywhere on the page and select the “View Page Source” option (“Show Page Source” in Safari).

Another tab should open with the raw HTML that makes this page. See if you can locate its various elements, and this challenge box in particular.

Using the Safari browser

If you are using Safari, you must first turn on the “Develop” menu in order to view the page source, and use the functions that we will use later in this section. To do so, navigate to Safari > Preferences and in the Advanced tab select the “Show Develop in menu bar” option. Note: In recent versions of Safari you must first turn on the “Develop” menu (in Preferences) and then navigate to Develop > Show Javascript Console and then click on the “Console” tab.

The HTML structure of the page you are currently reading looks something like this (most text and elements have been removed for clarity):

<!doctype html>
<html lang="en">
  <head>
    (...)
    <title>Selecting content on a web page with XPath</title>
  </head>
  <body>
	 (...)
  </body>
</html>

We can see from the source code that the title of this page is in a title element that is itself inside the head element, which is itself inside an html element that contains the entire content of the page.

Say we wanted to tell a web scraper to look for the title of this page, we would use this information to indicate the path the scraper would need to follow at it navigates through the HTML content of the page to reach the title element. XPath allows us to do that.

We can run XPath queries directly from within all major modern browsers, by enabling the built-in JavaScript console.

Display the console in your browser

  • In Firefox, use to the Tools > Web Developer > Web Console menu item.
  • In Chrome, use the View > Developer > JavaScript Console menu item.
  • In Safari, use the Develop > Show Error Console menu item. If your Safari browser doesn’t have a Develop menu, you must first enable this option in the Preferences, see above.

Here is how the console looks like in the Firefox browser:

JavaScript console in Firefox

For now, don’t worry too much about error messages if you see any in the console when you open it. The console should display a prompt with a > character (>> in Firefox) inviting you to type commands.

The syntax to run an XPath query within the JavaScript console is $x("XPATH_QUERY"), for example:

$x("/html/head/title/text()")

This should return something similar to

<- Array [ #text "Selecting content on a web page with XPath" ]

The output can vary slightly based on the browser you are using. For example in Chrome, you have to “open” the return object by clicking on it in order to view its contents.

Let’s look closer at the XPath query used in the example above: /html/head/title/text(). The first / indicates the root of the document. With that query, we told the browser to

/ Start at the root of the document…
html/ … navigate to the html node …
head/ … then to the head node that’s inside it…
title/ … then to the title node that’s inside it…
text() and select the text node contained in that element

Using this syntax, XPath thus allows us to determine the exact path to a node.

Select the “Introduction” title

Write an XPath query that selects the “Introduction” title above and try running it in the console.

Tip: if a query returns multiple elements, the syntax element[1] can be used. Note that XPath uses one-based indexing, therefore the first element has index 1, the second has index 2 etc.

Solution

$x("/html/body/div/article/h1[1]")

should produce something similar to

<- Array [ <h1#introduction> ]

Before we look into other ways to reach a specific HTML node using XPath, let’s start by looking closer at how nodes are arranged within a document and what their relationships with each others are.

For example, to select all the blockquote nodes of this page, we can write

$x("/html/body/div/article/blockquote")

This produces an array of objects:

<- Array [ <blockquote.objectives>, <blockquote.callout>, <blockquote.callout>, <blockquote.challenge>, <blockquote.callout>, <blockquote.callout>, <blockquote.challenge>, <blockquote.challenge>, <blockquote.challenge>, <blockquote.keypoints> ]

This selects all the blockquote elements that are under html/body/div. If we want instead to select all blockquote elements in this document, we can use the // syntax instead:

$x("//blockquote")

This produces a longer array of objects:

<- Array [ <blockquote.objectives>, <blockquote.callout>, <blockquote.callout>, <blockquote.challenge>, <blockquote.callout>, <blockquote.callout>, <blockquote.challenge>, <blockquote.solution>, <blockquote.challenge>, <blockquote.solution>, 3 more… ]

Why is the second array longer?

If you look closely into the array that is returned by the $x("//blockquote") query above, you should see that it contains objects like <blockquote.solution> that were not included in the results of the first query. Why is this so?

Tip: Look at the source code and see how the challenges and solutions elements are organised.

We can use the class attribute of certain elements to filter down results. For example, looking at the list of blockquote elements returned by the previous query, and by looking at this page’s source, we can see that the blockquote elements on this page are of different classes (challenge, solution, callout, etc.).

To refine the above query to get all the blockquote elements of the challenge class, we can type

$x("//blockquote[@class='challenge']")

which returns

Array [ <blockquote.challenge>, <blockquote.challenge>, <blockquote.challenge>, <blockquote.challenge> ]

Select the “Introduction” title by ID

In a previous challenge, we were able to select the “Introduction” title because we knew it was the first h1 element on the page. But what if we didn’t know how many such elements were on the page. In other words, is there a different attribute that allows us to uniquely identify that title element?

Using the path expressions introduced above, rewrite your XPath query to select the “Introduction” title without using the [1] index notation.

Tips:

  • Look at the source of the page or use the “Inspect element” function of your browser to see what other information would enable us to uniquely identify that element.
  • The syntax for selecting an element like <div id="mytarget"> is div[@id = 'mytarget'].

Solution

$x("/html/body/div/h1[@id='introduction']")

should produce something similar to

<- Array [ <h1#introduction> ]

Select this challenge box

Using an XPath query in the JavaScript console of your browser, select the element that contains the text you are currently reading on this page.

Tips:

  • In principle, id attributes in HTML are unique on a page. This means that if you know the id of the element you are looking for, you should be able to construct an XPath that looks for this value without having to worry about where in the node tree the target element is located.
  • The syntax for selecting an element like <div id="mytarget"> is div[@id = 'mytarget'].
  • Remember that XPath queries are relative to a context node, and by default that node is the root node.
  • Use the // syntax to select for elements regardless of where they are in the tree.
  • The syntax to select the parent element relative to a context node is ..
  • The $x(...) JavaScript syntax will always return an array of nodes, regardless of the number of nodes returned by the query. Contrary to XPath, JavaScript uses zero based indexing, so the syntax to get the first element of that array is therefore $x(...)[0].

Make sure you select this entire challenge box. If the result of your query displays only the title of this box, have a second look at the HTML structure of the document and try to figure out how to “expand” your selection to the entire challenge box.

Solution

Let’s have a look at the HTML code of this page, around this challenge box (using the “View Source” option) in our browser). The code looks something like this:

<!doctype html>
<html lang="en">
  <head>
    (...)
  </head>
  <body>
	<div class="container">
	(...)
	  <blockquote class="challenge">
	    <h2 id="select-this-challenge-box">Select this challenge box</h2>
	    <p>Using an XPath query in the JavaScript console of your browser...</p>
	    (...)
	  </blockquote>
	(...)
	</div>
  </body>
</html>

We know that the id attribute should be unique, so we can use this to select the h2 element inside the challenge box:

$x("//h2[@id = 'select-this-challenge-box']/..")[0]

This should return something like

<- <blockquote class="challenge">

Let’s walk through that syntax:

$x(" This function tells the browser we want it to execute an XPath query.
// Look anywhere in the document…
h2 … for an h2 element …
[@id = 'select-this-challenge-box'] … that has an id attribute set to select-this-challenge-box
.. and select the parent node of that h2 element
")" This is the end of the XPath query.
[0] Select the first element of the resulting array (since $x() returns an array of nodes and we are only interested in the first one).

By hovering on the object returned by your XPath query in the console, your browser should helpfully highlight that object in the document, enabling you to make sure you got the right one:

Hovering over a resulting node in Firefox

Advanced XPath syntax

FIXME: All the content below is from the original XPath lesson. Adapt content to use current example.

Operators

Operators are used to compare nodes. There are mathematical operators, boolean operators. Operators can give you boolean (true/false values) as a result. Here are some useful ones:

Operator Explanation
= Equivalent comparison, can be used for numeric or text values
!= Is not equivalent comparison
>, >= Greater than, greater than or equal to
<, <= Less than, less than or equal to
or Boolean or
and Boolean and
not Boolean not

Examples

Path Expression Expression Result
html/body/div/h3/@id=’exercises-2’ Does exercise 2 exist?
html/body/div/h3/@id!=’exercises-4’ Does exercise 4 not exist?
//h1/@id=’references’ or @id=’introduction’ Is there an h1 references or introduction?

Predicates

Predicates are used to find a specific node or a node that contains a specific value.

Predicates are always embedded in square brackets, and are meant to provide additional filtering information to bring back nodes. You can filter on a node by using operators or functions.

Examples

Operator Explanation
[1] Select the first node
[last()] Select the last node
[last()-1] Select the last but one node (also known as the second last node)
[position()<3] Select the first two nodes, note the first position starts at 1, not =
[@lang] Select nodes that have attribute ‘lang’
[@lang='en'] Select all the nodes that have a “attribute” attribute with a value of “en”
[price>15.00] Select all nodes that have a price node with a value greater than 15.00

Examples

Path Expression Expression Result
//h1[2] Select 2nd h1
//h1[@id=’references’ or @id=’introduction’] Select h1 references or introduction

Wildcards

XPath wildcards can be used to select unknown XML nodes.

Wildcard Description
* Matches any element node
@* Matches any attribute node
node() Matches any node of any kind

Examples

Path Expression Result //*[@id=”examples-2”]
//*[@class='solution'] Select all elements with class attribute ‘solution’  

XPath can do in-text searching using functions and also supports regex with its matches() function. Note: in-text searching is case-sensitive!

Path Expression Result
//author[contains(.,"Matt")] Matches on all author nodes, in current node contains Matt (case-sensitive)
//author[starts-with(.,"G")] Matches on all author nodes, in current node starts with G (case-sensitive)
//author[ends-with(.,"w")] Matches on all author nodes, in current node ends with w (case-sensitive)
//author[matches(.,"Matt.*")] regular expressions match 2.0

Complete syntax: XPath Axes

XPath Axes fuller syntax of how to use XPath. Provides all of the different ways to specify the path by describing more fully the relationships between nodes and their connections. The XPath specification describes 13 different axes:

XPath Axes Image Credit: SAMS Teach Yourself XSLT in 21 Days

Path Expression Result
/html/body/div/h1[@id='introduction']/following-sibling::h1 Select all h1 following siblings of the h1 introduction
/html/body/div/h1[@id='introduction']/following-sibling::* Select all h1 following siblings
//attribute::id Select all id attribute nodes

Oftentimes, the elements we are looking for on a page have no ID attribute or other uniquely identifying features, so the next best thing is to aim for neighboring elements that we can identify more easily and then use node relationships to get from those easy to identify elements to the target elements.

For example, the node tree image above has no uniquely identifying feature like an ID attribute. However, it is just below the section header “Navigating through the HTML node tree using XPath”. Looking at the source code of the page, we see that that header is a h2 element with the id navigating-through-the-html-node-tree-using-xpath.

$x("//h2[@id='navigating-through-the-html-node-tree-using-xpath']/following-sibling::p[2]/img")

Additions

FIXME: add more XPath functions such as concat() and normalize-space(). FIXME: mention XPath Checker for Firefox FIXME: Firefox sometime cleans up the HTML of a page before displaying it, meaning that the DOM tree we can access through the console might not reflect the actual source code. <tbody> elements are typically not reliable. The Scrapy documentation has more on the topic.

References

Key Points

  • XML and HTML are markup languages. They provide structure to documents.

  • XML and HTML documents are made out of nodes, which form a hierarchy.

  • The hierarchy of nodes inside a document is called the node tree.

  • Relationships between nodes are: parent, child, sibling.

  • XPath queries are constructed as paths going up or down the node tree.

  • XPath queries can be run in the browser using the $x() function.