Test Migration
Migrating tests from Python pytest to Rust #[test] is as important as migrating the code itself. This chapter maps common pytest patterns to their Rust equivalents.
Pytest to Rust Mapping
| pytest Pattern | Rust Equivalent |
|---|---|
def test_foo(): | #[test] fn test_foo() |
assert x == y | assert_eq!(x, y) |
with pytest.raises(ValueError): | #[should_panic] or assert!(result.is_err()) |
@pytest.fixture | Helper function or LazyLock |
@pytest.mark.parametrize | test-case crate or proptest! |
conftest.py | mod test_helpers |
tmpdir fixture | tempfile::TempDir |
Fixture Patterns
# Python
@pytest.fixture
def sample_model():
return Model(layers=4, hidden=256)
#![allow(unused)]
fn main() {
// Rust: helper function
fn sample_model() -> Model {
Model::new(4, 256)
}
// Rust: lazy static for expensive setup
use std::sync::LazyLock;
static SAMPLE_MODEL: LazyLock<Model> = LazyLock::new(|| Model::new(4, 256));
}Parameterized Tests
#![allow(unused)]
fn main() {
use test_case::test_case;
#[test_case(1, 2 ; "one")]
#[test_case(3, 6 ; "three")]
#[test_case(5, 10 ; "five")]
fn test_double(input: i32, expected: i32) {
assert_eq!(double(input), expected);
}
}Error Testing
#![allow(unused)]
fn main() {
#[test]
fn test_invalid_input() {
let result = compute(-1);
assert!(result.is_err());
assert!(result.unwrap_err().to_string().contains("negative"));
}
}Temporary Files
#![allow(unused)]
fn main() {
#[test]
fn test_save_load() {
let dir = tempfile::tempdir().unwrap();
let path = dir.path().join("model.bin");
save(&model, &path).unwrap();
let loaded = load(&path).unwrap();
// dir cleaned up on drop
}
}Migration Checklist
- Inventory all pytest files and count test functions
- Map fixtures to Rust helpers (create
test_helpers.rs) - Convert assertions one file at a time
- Run both test suites during migration to catch gaps
- Remove Python tests only after Rust coverage meets 95%
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