parameterized-tests.md

Parameterized Tests

When testing the same behavior with different inputs and expected outputs, prefer parameterized tests over loops in the test body.

Why? Parameterized tests provide:

• Better Test Isolation: Each parameter combination runs as a separate test case
• Clearer Test Output: Individual test cases show up separately in test results
• Parallel Execution: Test framework can run each case in parallel
• Easier Debugging: When a test fails, you know exactly which parameter combination caused it
• Better IDE Support: Modern IDEs can run individual parameterized test cases

How? Use the rstest crate for parameterized testing.

❌ Avoid: Loop in Test Body

#[test]
fn it_should_create_state_file_in_environment_specific_subdirectory() {
    let test_cases = vec![
        ("e2e-config", "e2e-config/state.json"),
        ("e2e-full", "e2e-full/state.json"),
        ("e2e-provision", "e2e-provision/state.json"),
    ];

    for (env_name, expected_path) in test_cases {
        // Test logic here...
        // If one case fails, you don't know which one without debugging
    }
}

Problem: If the second iteration fails, the test output only shows the test name, not which specific case failed.

✅ Good: Parameterized Test with rstest

use rstest::rstest;

#[rstest]
#[case("e2e-config", "e2e-config/state.json")]
#[case("e2e-full", "e2e-full/state.json")]
#[case("e2e-provision", "e2e-provision/state.json")]
fn it_should_create_state_file_in_environment_specific_subdirectory(
    #[case] env_name: &str,
    #[case] expected_path: &str,
) {
    // Test logic here...
    // Each case runs as a separate test with clear identification
}

Benefits: Test output shows individual cases:

• it_should_create_state_file_in_environment_specific_subdirectory::case_1 ✅
• it_should_create_state_file_in_environment_specific_subdirectory::case_2 ✅
• it_should_create_state_file_in_environment_specific_subdirectory::case_3 ✅

When to Use Parameterized Tests

Use parameterized tests when:

• ✅ Testing the same behavior with multiple input/output combinations
• ✅ Validating edge cases with different values
• ✅ Testing configuration variations
• ✅ Verifying data transformation with various inputs

Don't use parameterized tests when:

• ❌ Each case tests fundamentally different behavior (use separate tests)
• ❌ The test logic differs significantly between cases
• ❌ You only have one or two cases (just write separate tests)

Setup

Add rstest to your Cargo.toml:

[dev-dependencies]
rstest = "0.23"

Then import it in your test module:

#[cfg(test)]
mod tests {
    use rstest::rstest;
    // ... other imports
}