Go Test Main

In this tutorial, you'll learn about Go Test: TestMain Setup. We cover key concepts, practical examples, and best practices.

TestMain -- Use TestMain to set up and tear down package-level test fixtures like databases and servers.

The Problem

TestMain runs once per package before and after all tests. Use it for expensive setup that should not be repeated per test.

Wrong

// Setup repeated across many tests:
func TestA(t *testing.T) {
    db := setupDB() // Repeated
    defer db.Close()
}
func TestB(t *testing.T) {
    db := setupDB() // Repeated
    defer db.Close()
}

Output:

// Setup runs for every test. Slow.

Right

func TestMain(m *testing.M) {
    // Setup
    db := setupDB()
    code := m.Run() // Run all tests
    // Teardown
    db.Close()
    os.Exit(code)
}

Output:

// One-time setup and teardown for all tests in package

Prevention

• Define TestMain(m *testing.M) in one file per package
• Call m.Run() to execute all tests
• os.Exit(m.Run()) for proper exit code
• One-time setup: DB connection, test server, test data
• Cleanup after m.Run() returns

Common Mistakes with test main

1. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
2. Non-exhaustive pattern matches that compile with warnings then crash at runtime
3. Misunderstanding that String is [Char] with poor performance for large text operations

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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