At what level of hierarchy do you begin your selectors? There seems to be a convention of beginning with the container of the target element, but why not ever the target element itself, especially in the case of an id or starting with a wildcard plus a unique identifier? Recursive descent seems like everyone's best friend.
1 Answer
XPaths and Css-Selectors are very versatile, and can describe the same element in many different ways - i.e. an single element has infinitely many possible locators to describe it. The goal is to get something to fit the needs of the developer which might include being readable, unique, and or adaptive.
Consider the following html example:
<div id='mainContainer'>
<span>some span</span>
</div>
If I were trying to make a locator for the <span> element, I wouldn't choose //span, because that will probably yield way too many results. Instead you could start with its parent who has an id, and then proceed to the span: //*[@id='mainContainer']/span, and alternatively: //span[parent::*[@id='mainContainer']]. Which XPath is better? Whichever one you personally find more readable. I agree with you that the first example does seem to be more common, although I myself am more partial to the latter.
Sometimes the point of making a locator a certain way is to be adaptable. For instance, I rarely write a locator like this: //*[@class='fooBar']. The reason is because in modern web development classes come and go frequently, and it's likely that that element's class could change at the slightest breeze. Instead you might write //*[contains(@class,'fooBar')]. Now when a developer goes in and adds a class for pure styling, you don't have to go back and update all of your selenium tests. That is also the reason I use wildcard characters frequently. If a developer goes in and updates a div to a span, my test will still work.
As @Gilles Quenot commented, it isn't always safe to assume that ids are unique. Many websites were written by someone's unemployed uncle who took an html class back in '86. They are terrible, and don't care at all about standards or audits. This is another reason that you need to include enough information in your locator to specify the exact element/elements you are talking about, but not too much information that you are describing too many elements.
One more comment is that XPaths are bidirectional, whereas Css-Selectors are not. This means XPaths can go from child to parent and from parent to child, where Css-Selectors can only go from parent to child. This affects which node you are starting at, and may be a reason that you see more Css-Selectors start from a parent/ancestor node.
TL;DR There isn't a convention, just personal preferences. Do what meets your needs.
#foobaror in xpath//*[@id="foobar"]