Unit test

As you build the logic for your runtime, you'll want to routinely test that the logic works as expected. You can create unit test for the runtime using the unit testing framework provided by Rust. After you create one or more unit tests, you can use the cargo test command to execute the test. For example, you can run all of the tests you have created for a runtime by running the following command:

cargo test

For more information about using the Rust cargo test command and testing framework, run the following command:

cargo help test

Test pallet log in a mock runtime

In addition to the unit testing you can do with the Rust testing framework, you can verify the logic in your runtime by constructing a mock runtime environment. The configuration type Test is defined as a Rust enum with implementations for each of the pallet configuration traits that are used in the mock runtime.

 pub enum Test where
  Block = Block,
  NodeBlock = Block,
  UncheckedExtrinsic = UncheckedExtrinsic,
  System: frame_system::{Pallet, Call, Config, Storage, Event<T>},
  TemplateModule: pallet_template::{Pallet, Call, Storage, Event<T>},

impl frame_system::Config for Test {
 // -- snip --
 type AccountId = u64;

If Test implements pallet_balances::Config, the assignment might use u64 for the Balance type. For example:

impl pallet_balances::Config for Test {
 // -- snip --
 type Balance = u64;

By assigning pallet_balances::Balance and frame_system::AccountId to u64, testing accounts and balances only requires tracking a (AccountId: u64, Balance: u64) mapping in the mock runtime.

Test storage in a mock runtime

The sp-io crate exposes a TestExternalities implementation that you can use to test storage in a mock environment. It is the type alias for an in-memory, hashmap-based externalities implementation in substrate_state_machine referred to as TestExternalities.

The following example demonstrates defining a struct called ExtBuilder to build an instance of TestExternalities, and setting the block number to 1.

pub struct ExtBuilder;

impl ExtBuilder {
 pub fn build(self) -> sp_io::TestExternalities {
  let mut t = system::GenesisConfig::default().build_storage::<TestRuntime>().unwrap();
  let mut ext = sp_io::TestExternalities::new(t);
  ext.execute_with(|| System::set_block_number(1));

To create the test environment in unit tests, the build method is called to generate a TestExternalities using the default genesis configuration.

fn fake_test_example() {
 ExtBuilder::default().build_and_execute(|| {
  // ...test logics...

Custom implementations of Externalities allow you to construct runtime environments that provide access to features of the outer node. Another example of this can be found in the offchain module. The offchain module maintains its own Externalities implementation.

Genesis config

In the previous example, the ExtBuilder::build() method used the default genesis configuration for building the mock runtime environment. In many cases, it is convenient to set storage before testing. For example, you might want to pre-seeding account balances before testing.

In the implementation of frame_system::Config, AccountId and Balance are both set to u64. You can put (u64, u64) pairs in the balances vec to seed (AccountId, Balance) pairs as the account balances. For example:

impl ExtBuilder {
 pub fn build(self) -> sp_io::TestExternalities {
  let mut t = frame_system::GenesisConfig::default().build_storage::<Test>().unwrap();
  pallet_balances::GenesisConfig::<Test> {
   balances: vec![
    (1, 10),
    (2, 20),
    (3, 30),
    (4, 40),
    (5, 50),
    (6, 60)
   .assimilate_storage(&mut t)

  let mut ext = sp_io::TestExternalities::new(t);
  ext.execute_with(|| System::set_block_number(1));

AIn this example, acount 1 has balance 10, account 2 has balance 20, and so on.

The exact structure used to define the genesis configuration of a pallet depends on the pallet GenesisConfig struct definition. For example, in the Balances pallet, it is defined as:

pub struct GenesisConfig<T: Config<I>, I: 'static = ()> {
 pub balances: Vec<(T::AccountId, T::Balance)>,

Block production

It is useful to simulate block production to verify that expected behavior holds across block production.

A simple way of doing this is by incrementing the System module's block number between on_initialize and on_finalize calls from all modules with System::block_number() as the sole input. Although it is important for runtime code to cache calls to storage or the system module, the test environment scaffolding should prioritize readability to facilitate future maintenance.

fn run_to_block(n: u64) {
 while System::block_number() < n {
  if System::block_number() > 1 {
  System::set_block_number(System::block_number() + 1);

The on_finalize and on_initialize methods are only called from ExamplePallet if the pallet trait implements the frame_support::traits::{OnInitialize, OnFinalize} traits to execute the logic encoded in the runtime methods before and after each block respectively.

Then call this function in the following fashion.

fn my_runtime_test() {
 with_externalities(&mut new_test_ext(), || {

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