Go Test: Helper Functions Hide Line Numbers

In this tutorial, you'll learn about Go Test: Helper Functions Hide Line Numbers. We cover key concepts, practical examples, and best practices.

Test helper functions -- Use t.Helper() to mark helper functions so test failures show the correct caller line.

The Problem

Without t.Helper(), a failed assertion in a helper function reports the line inside the helper, not the test function that called it. t.Helper() fixes this.

Wrong

func assertEqual(t *testing.T, got, want int) {
    if got != want {
        t.Errorf("got %d, want %d", got, want) // Line inside helper!
    }
}
func TestAdd(t *testing.T) {
    assertEqual(t, add(2, 3), 5)
}

Output:

// Error shows line in assertEqual, not TestAdd

Right

func assertEqual(t *testing.T, got, want int) {
    t.Helper() // Marks this as helper
    if got != want {
        t.Errorf("got %d, want %d", got, want)
    }
}
func TestAdd(t *testing.T) {
    assertEqual(t, add(2, 3), 5)
}

Output:

// Error shows line in TestAdd, where assertEqual was called

Prevention

• Call t.Helper() at the start of helper functions
• Works for Errorf, Fatalf, Fatal, Error
• Chainable: call t.Helper() in nested helpers too
• Absolutely use for table-driven test runners
• Greatly improves debugging test failures

Common Mistakes with test helper func

1. Misunderstanding that String is [Char] with poor performance for large text operations
2. Using foldl instead of foldl' causing stack overflow on large lists
3. Forgetting deriving (Show, Eq) on custom data types needed for debugging

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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