Understanding Common Errors

Invalid Selector Exception

CSS and XPath Selectors are sometimes difficult to get correct.

Likely Cause

The CSS or XPath selector you are trying to use has invalid characters or an invalid query.

Possible Solutions

Run your selector through a validator service:

• CSS Validator
• xPath Validator

Or use a browser extension to get a known good value:

• SelectorsHub

No Such Element Exception

The element can not be found at the exact moment you attempted to locate it.

Likely Cause

• You are looking for the element in the wrong place (perhaps a previous action was unsuccessful).
• You are looking for the element at the wrong time (the element has not shown up in the DOM, yet)
• The locator has changed since you wrote the code

Possible Solutions

• Make sure you are on the page you expect to be on, and that previous actions in your code completed correctly
• Make sure you are using a proper Waiting Strategy
• Update the locator with the browser’s devtools console or use a browser extension like:
  • SelectorsHub

Stale Element Reference Exception

An element goes stale when it was previously located, but can not be currently accessed. Elements do not get relocated automatically; the driver creates a reference ID for the element and has a particular place it expects to find it in the DOM. If it can not find the element in the current DOM, any action using that element will result in this exception.

Common Causes

This can happen when:

• You have refreshed the page, or the DOM of the page has dynamically changed.
• You have navigated to a different page.
• You have switched to another window or into or out of a frame or iframe.

Common Solutions

The DOM has changed

When the page is refreshed or items on the page have moved around, there is still an element with the desired locator on the page, it is just no longer accessible by the element object being used, and the element must be relocated before it can be used again. This is often done in one of two ways:

• Always relocate the element every time you go to use it. The likelihood of the element going stale in the microseconds between locating and using the element is small, though possible. The downside is that this is not the most efficient approach, especially when running on a remote grid.

• Wrap the Web Element with another object that stores the locator, and caches the located Selenium element. When taking actions with this wrapped object, you can attempt to use the cached object if previously located, and if it is stale, exception can be caught, the element relocated with the stored locator, and the method re-tried. This is more efficient, but it can cause problems if the locator you’re using references a different element (and not the one you want) after the page has changed.

The Context has changed

Element objects are stored for a given context, so if you move to a different context — like a different window or a different frame or iframe — the element reference will still be valid, but will be temporarily inaccessible. In this scenario, it won’t help to relocate the element, because it doesn’t exist in the current context. To fix this, you need to make sure to switch back to the correct context before using the element.

The Page has changed

This scenario is when you haven’t just changed contexts, you have navigated to another page and have destroyed the context in which the element was located. You can’t just relocate it from the current context, and you can’t switch back to an active context where it is valid. If this is the reason for your error, you must both navigate back to the correct location and relocate it.

Invalid SessionId Exception

Sometimes the session you’re trying to access is different than what’s currently available

Likely Cause

This usually occurs when the session has been deleted (e.g. driver.quit()) or if the session has changed, like when the last tab/browser has closed (e.g. driver.close())

Possible Solutions

Check your script for instances of driver.close() and driver.quit(), and any other possible causes of closed tabs/browsers. It could be that you are locating an element before you should/can.