cssSelector vs XPath for selenium

Question

As per my understanding, CSS selector traverses through the DOM. Because CSS files will not have any info about element position then why cssSelector is faster then

Answer 1

The debate between cssSelector vs XPath still now remains as one of the most heated and subjective conversation in the Selenium Community. A quick recap on what we had already known so far can be summarized as:

• People in favor of cssSelector say that it is more readable and faster (specifically when running against Internet Explorer).
• While those in favor of XPath tout it's ability to transverse the page (while cssSelector cannot).
• Traversing the DOM in older browsers like IE8 does not work with cssSelector but is fine with XPath.
• XPath can walk up the DOM (e.g. from child to parent), whereas cssSelector can only traverse down the DOM (e.g. from parent to child)
• However not being able to traverse the DOM with cssSelector in older browsers isn't necessarily a bad thing as it is more of an indicator that your page has poor design and could benefit from some helpful markup.

Dave Haeffner carried out a test on a page with two HTML data tables, one table is written without helpful attributes (ID and Class), and the other with them. I have analyzed the test procedure and the outcome of this experiment in details in the discussion Why should I ever use CSS selectors as opposed to XPath for automated testing?. While this experiment demonstrated that each Locator Strategy is reasonably equivalent across browsers, it didn't adequately paint the whole picture for us.

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cssSelector vs XPath, Under a Microscope

Dave Haeffner in the discussion Css Vs. X Path, Under a Microscope mentioned, in an an end-to-end test there were a lot of other variables at play Sauce startup, Browser start up, and latency to and from the application under test. The unfortunate takeaway from that experiment could be that one driver may be faster than the other (e.g. IE vs Firefox), when in fact, that's wasn't the case at all. To get a real taste of what the performance difference is between cssSelector and XPath, we need to dig deeper. This can be achieved by running everything from a local machine while using a performance bench-marking utility. The focus was on a specific Selenium action rather than the entire test run, and run things numerous times.

To demonstrate this detailed example, a Windows XP virtual machine was setup and Ruby (1.9.3) was installed. All the available browsers and their equivalent browser drivers for Selenium was also installed. For bench-marking, Ruby's standard lib benchmark was used.

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The Test

In order to get an adequate sample set of data, the same test was ran against each browser 100 times. And to weed out anomalies in the data the rehearsal feature of benchmark was used so that it would run the full test sequence, perform garbage collection, and then run it again. To make things comparable, a few of the locators were updated to make for better matches in comparison to each other. And the specific action we measured is find_element.

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Test Code

require_relative 'base'
require 'benchmark'

class SmallDOM < Base

  LOCATORS = {
    :id => {
      id: 'table2'
    },
    :table_header_class => {
      class: 'dues'
    },
    :table_header_id_and_class => {
      :css => "#table2 thead .dues",
      :xpath => "//table[@id='table2']//thead//*[@class='dues']"
    },
    :table_header_id_class_and_direct_desc => {
      :css => "#table2 > thead .dues",
      :xpath => "//table[@id='table2']/thead//*[@class='dues']"
    },
    :table_header_traversing => {
      :css => "#table2 thead tr th:nth-of-type(4)",
      :xpath => "//table[@id='table2']//thead//tr//th[4]"
    },
    :table_header_traversing_and_direct_desc => {
      :css => "#table2 > thead > tr > th:nth-of-type(4)",
      :xpath => "//table[@id='table2']/thead/tr/th[4]"
    },
    :table_cell_id_and_class => {
      :css => "#table2 tbody .dues",
      :xpath => "//table[@id='table2']//tbody//*[@class='dues']"
    },
    :table_cell_id_class_and_direct_desc => {
      :css => "#table2 > tbody .dues",
      :xpath => "//table[@id='table2']/tbody//*[@class='dues']"
    },
    :table_cell_traversing => {
      :css => "#table2 tbody tr td:nth-of-type(4)",
      :xpath => "//table[@id='table2']//tbody//tr//td[4]"
    },
    :table_cell_traversing_and_direct_desc => {
      :css => "#table2 > tbody > tr > td:nth-of-type(4)",
      :xpath => "//table[@id='table2']/tbody/tr/td[4]"
    }
  }

  attr_reader :driver

  def initialize(driver)
    @driver = driver
    visit '/tables'
    super
  end

  # The benchmarking approach was borrowed from
  # http://rubylearning.com/blog/2013/06/19/how-do-i-benchmark-ruby-code/
  def benchmark
    Benchmark.bmbm(27) do |bm|
      LOCATORS.each do |example, data|
    data.each do |strategy, locator|
      bm.report(example.to_s + " using " + strategy.to_s) do
        begin
          ENV['iterations'].to_i.times do
         find(strategy => locator)
          end
        rescue Selenium::WebDriver::Error::NoSuchElementError
          puts "( 0.0 )"
        end
      end
    end
      end
    end
  end

end

Results

NOTE: The output is in seconds, and the results are for the total run time of 100 executions.

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Analyzing the Results

• On a whole, Internet Explorer is slower than the other drivers, but between CSS and XPath it looks like XPath is actually faster than CSS.
• Chrome and Opera have some differences, albeit much smaller, but they sway in both directions.
• In some cases CSS is faster, and in others, XPath.
• Firefox looks to be a bit more optimized for CSS since it's mostly faster across the board.

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Outro

Even with these speed differences they are only a few seconds (or fractions of seconds) apart -- and that's for 100 executions. When you think about how it takes 30 seconds or more to complete a test run, this kind of difference is negligible. So, the choice between css-selectors and xpath can be a tough one to make. But now you are armed with more than enough data to make the choice for yourself. It's really just a matter of finding what works for you and your team and to not get weighed down by the hype and opinions around which one is better.