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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.

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