What good practices and anti-patterns are there in unit testing? How to understand that tests are good?

Good practices for unit testing:

• Isolation: each test checks only one specific aspect or function.
• Repeatability: tests should give the same result every time they are run, regardless of the environment.
• Performance: unit tests should execute quickly so as not to slow down the development process.
• Clarity: test code should be simple and readable to easily understand what is being tested.
• Use of mocks and stubs: to isolate the tested code from external dependencies.
• Boundary coverage: tests should check not only normal scenarios but also edge cases.

Anti-patterns:

• Testing multiple things at once: complicates understanding and debugging.
• Dependence on external resources: databases, networks, etc., make tests unstable.
• Complex and lengthy tests: hinder maintenance.
• Lack of cleanup after tests: can affect subsequent tests.
• Testing private methods directly: better to test through the public interface.

How to understand if tests are good:

• They run quickly and pass reliably.
• Easy to understand what exactly is being tested.
• When code changes, tests help identify regressions.
• Cover main and boundary cases.

Example of a simple unit test in Go:

func Add(a, b int) int {
    return a + b
}

func TestAdd(t *testing.T) {
    result := Add(2, 3)
    if result != 5 {
        t.Errorf("Add(2, 3) = %d; want 5", result)
    }
}