Why is it important for tests to be isolated?

It is important for tests to be isolated because each test should check only one specific thing and not depend on the results of other tests. If tests depend on each other or affect each other, they become fragile, unpredictable and poorly reflect the actual quality of the code.

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#The main reasons isolation matters

1) Reliability An isolated test either passes or fails for one clear reason. If tests are linked, one failure can trigger a "domino effect," making it impossible to quickly understand where the real error is.

2) Predictability Isolated tests always give the same result on a repeated run. If a test depends on state, data or other tests, the result becomes random: green today, red tomorrow.

3) Easy debugging When a test checks exactly one idea and relies on nothing else, it is much easier to find the source of the error. The developer immediately sees what broke, instead of untangling a chain of dependencies.

4) Parallel runs In many projects, tests run in multiple threads. If tests are not isolated and share common state, running them in parallel leads to conflicts and false failures.

5) Fewer false positives Isolation eliminates situations where a test fails not because of a bug, but because of "dirty" state left behind by a previous test.

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#The main idea

An isolated test is a test that nothing hinders and that needs nobody else to check its own behavior. It is exactly such tests that provide stability, accuracy and trust in the results.