Which characteristic of effective unit tests ensures they can be run in isolation?

The characteristic that ensures unit tests can be run in isolation is independence. When unit tests are independent, it means they do not rely on the outcomes or states produced by other tests. This allows each test to be executed in isolation, ensuring that it only verifies a specific piece of functionality or logic without any external dependencies affecting the results.

Independence is crucial because it minimizes interference and variability between tests, making it easier to identify failures or issues in specific areas of the code. When tests are independent, developers can run them at any time and on any machine without having to worry about the order in which they are executed or the state of the system. This reliability enhances the developer's ability to maintain and refactor code confidently, knowing that failures in tests reliably point to issues in the specific unit being tested.

The other characteristics, such as readability, accuracy, and speed, although important for unit tests overall, do not specifically address the requirement for tests to run in isolation. Readable tests help developers understand what is being tested and why. Accurate tests ensure that the correct functionality is being verified. Fast tests allow for quick feedback during development, encouraging frequent execution. However, these traits do not directly ensure that tests can be run without dependence on one another.