Putting integers into quotes for xpath

Question

I was debugging someone else's Selenium code. They had an xpath something like td['6']. This was failing. I used intuition and changed it to td[6] which fixed it. However, td['6'] did not give an error as I thought it would. It located an element, but a completely different one than without the quotes.

So it got me to thinking, what does putting the number in quotes, like td['6'] actually mean?

Answer 1

The XPath 1.0 Specification states that ’td’ is the axis (e.g. children) and node-test (tag name TD) an everything inside the square brackets is a predicate-expression which is either evaluated as boolean (true or false) or in special-case of a number evaluated as positional filter of the node-set:

if the result is not a number, then the result will be converted as if by a call to the boolean function. Thus a location path para[3] is equivalent to para[position()=3].

Explained by Example

Following 3 XPath predicate cases are explained by evaluating on input from this sample table (w3schools):

+-------------------------------+-------------------+---------+
|           Company             |     Contact       | Country |
+-------------------------------+-------------------+---------+
| Alfreds Futterkiste           | Maria Anders      | Germany |
| Centro comercial Moctezuma    | Francisco Chang   | Mexico  |
| Ernst Handel                  | Roland Mendel     | Austria |
| Island Trading                | Helen Bennett     | UK      |
| Laughing Bacchus Winecellars  | Yoshi Tannamuri   | Canada  |
| Magazzini Alimentari Riuniti  | Giovanni Rovelli  | Italy   |
+-------------------------------+-------------------+---------+

case 1: number as predicate

td[6] selects the 6th child table-data element since number 6 is evaluated as shorthand for predicate position()=6.

Example: select each cell of the the third column

XPath: //td[3]

Output:

  <td>Germany</td>  
  <td>Mexico</td>  
  <td>Austria</td>  
  <td>UK</td>  
  <td>Canada</td>  
  <td>Italy</td> 

Try online demo

case 2: quoted string as predicate

td['6'] selects every child table-data element since the string '6' is not-empty or has non-zero length and thus evaluates to true (see boolean-conversion). So node-set of TD-elements is not further filtered (because predicate is always true).

Example: select each cell (because the string-predicate is always true)

XPath: //td['3']

Output:

  <td>Alfreds Futterkiste</td>  
  <td>Maria Anders</td>  
  <td>Germany</td>  
  <td>Centro comercial Moctezuma</td>  
  <td>Francisco Chang</td>  
  <td>Mexico</td>  
  <td>Ernst Handel</td>  
  <td>Roland Mendel</td>  
  <td>Austria</td>  
  <td>Island Trading</td>  
  <td>Helen Bennett</td>  
  <td>UK</td>  
  <td>Laughing Bacchus Winecellars</td>  
  <td>Yoshi Tannamuri</td>  
  <td>Canada</td>  
  <td>Magazzini Alimentari Riuniti</td>  
  <td>Giovanni Rovelli</td>  
  <td>Italy</td> 

Try online demo

case 3: conditional predicate testing the elements

This is the real benefit of predicate expression and allows you to test on the elements, for example find all merged table-cells with colspan attribute:

td[@colspan]

See this sophisticated use-case: Xpath expression with multiple predicates

Example: select all cells where contents starts with 'A'

XPath: //tr/td[starts-with(., 'A')]

Output:

  <td>Alfreds Futterkiste</td>  
  <td>Austria</td>  

Try online demo

Answer 2

td[predicate] means:

return the first td node for which predicate is true.

Each non-empty string returns true, so td['6'] will select first td node found in DOM.

td[6] is the shorthand for td[position()=6] expression which means:

return td which is the sixth child of td type."