Test Organization Patterns in Go

Use internal test files (*_test.go) for unit tests within the same package and external test packages (*_test with a different package name) for integration tests or testing unexported symbols from outside the package.

// Internal test (same package, can access unexported symbols)
package mypkg

func TestInternal(t *testing.T) {
    // Access unexported helper() directly
    helper()
}

// External test (different package, only accesses exported symbols)
package mypkg_test

func TestExternal(t *testing.T) {
    // Can only call exported ExportedFunc()
    ExportedFunc()
}

Internal tests live in the same package as your code and can see private functions, making them ideal for detailed unit testing. External tests live in a separate package and can only see public functions, which is better for simulating how other developers will use your library. Think of internal tests as checking the engine while the car is in the shop, and external tests as driving the car on the road.