Testing Framework

The testing framework in this tutorial uses the Casper engine test support crate for testing the contract implementation against the Casper execution environment.

We will review the following three GitHub testing folders, which create a testing framework for the Casper ERC-20 project:

  • erc20-test-call - Links the test framework together and is required by the Rust toolchain

  • erc20-test - Sets up the testing context and creates helper functions used by unit tests

  • tests - Contains the unit tests

The following is an example of a complete test:

#[should_panic(expected = "ApiError::User(65534) [131070]")]
fn should_not_transfer_with_insufficient_balance() {
    let mut fixture = TestFixture::install_contract();

    let initial_ali_balance = fixture.balance_of(Key::from(fixture.ali)).unwrap();
    assert_eq!(fixture.balance_of(Key::from(fixture.bob)), None);

        initial_ali_balance + U256::one(),

To run the tests, issue the following command in the project folder, erc20:

make test

The project contains a Makefile, which is a custom build script that compiles the contract before running tests in release mode. Then, the script copies the contract.wasm file to the tests/wasm directory. In practice, you only need to run the make test command during development.

Configuring the Test Package

In this project, we define a tests package using the tests/Cargo.toml file.

name = "tests"
version = "0.1.0"

casper-types = "1.3.2"
casper-engine-test-support = "1.3.2"
casper-execution-engine = "1.3.2"
once_cell = "1.8.0"

name = "tests"

Testing Logic

In Github, you will find an example containing a Casper ERC-20 contract implementation with the corresponding tests. The tests follow this sequence:

  • Step 1: Specify the starting state of the blockchain.

  • Step 2: Deploy the compiled contract to the blockchain and query it.

  • Step 3: Create additional deploys for each of the methods in the contract.

The TestFixture accomplishes these steps by simulating a real-world deployment (storing the contract in the blockchain) and invoking methods in the contract.

Setting up the testing context

The code in the TestFixture initializes the blockchain’s global state with all the data and methods a smart contract needs.

Below is a subset of the required constants for this project. For the most up-to-date version of the code, visit GitHub.

// File https://github.com/casper-ecosystem/erc20/blob/master/example/erc20-tests/src/test_fixture.rs

use casper_engine_test_support::{Code, SessionBuilder, TestContext, TestContextBuilder};
use casper_erc20::constants as consts;
use casper_types::{
    bytesrepr::{FromBytes, ToBytes},
    runtime_args, AsymmetricType, CLTyped, ContractHash, Key, PublicKey, RuntimeArgs, U256, U512,

const CONTRACT_ERC20_TOKEN: &str = "erc20_token.wasm";
const CONTRACT_KEY_NAME: &str = "erc20_token_contract";

fn blake2b256(item_key_string: &[u8]) -> Box<[u8]> {...}

#[derive(Clone, Copy)]
pub struct Sender(pub AccountHash);

Deploying the contract

The next step is to define a struct that has its own virtual machine (VM) instance and implements the ERC-20 methods. This struct holds a TestContext of its own. The contract_hash and the session_code won’t change after the contract is deployed, so it is good to keep them handy.

This code snippet builds the context and includes the compiled contract .wasm binary being tested. The TestFixture struct creates a new instance of the CONTRACT_ERC20_TOKEN with the following accounts:

  • ali - account used to deploy the contract

  • bob

  • joe

Note: These accounts have a positive initial balance.

The full and most recent code implementation is available on GitHub.

// File https://github.com/casper-ecosystem/erc20/blob/master/example/erc20-tests/src/test_fixture.rs

pub struct TestFixture {
    context: TestContext,
    pub ali: AccountHash,
    pub bob: AccountHash,
    pub joe: AccountHash,

impl TestFixture {
    pub const TOKEN_NAME: &'static str = "Test ERC20";
    pub const TOKEN_SYMBOL: &'static str = "TERC";
    pub const TOKEN_DECIMALS: u8 = 8;
    const TOKEN_TOTAL_SUPPLY_AS_U64: u64 = 1000;

    pub fn token_total_supply() -> U256 {

    pub fn install_contract() -> TestFixture {
        let ali = PublicKey::ed25519_from_bytes([3u8; 32]).unwrap();
        let bob = PublicKey::ed25519_from_bytes([6u8; 32]).unwrap();
        let joe = PublicKey::ed25519_from_bytes([9u8; 32]).unwrap();

        let mut context = TestContextBuilder::new()
            .with_public_key(ali.clone(), U512::from(500_000_000_000_000_000u64))
            .with_public_key(bob.clone(), U512::from(500_000_000_000_000_000u64))

        let session_code = Code::from(CONTRACT_ERC20_TOKEN);
        let session_args = runtime_args! {
            consts::NAME_RUNTIME_ARG_NAME => TestFixture::TOKEN_NAME,
            consts::SYMBOL_RUNTIME_ARG_NAME => TestFixture::TOKEN_SYMBOL,
            consts::DECIMALS_RUNTIME_ARG_NAME => TestFixture::TOKEN_DECIMALS,
            consts::TOTAL_SUPPLY_RUNTIME_ARG_NAME => TestFixture::token_total_supply()

        let session = SessionBuilder::new(session_code, session_args)

        TestFixture {
            ali: ali.to_account_hash(),
            bob: bob.to_account_hash(),
            joe: joe.to_account_hash(),

Querying the network

The previous step has simulated a real deploy on the network. The next code snippet describes how to query the network to find the contract hash.

Contracts are deployed under the context of an account. Since we created the deploy under the context of self.ali, this is what we will query next. The query_contract function uses query to lookup named keys. It will be used to implement the balance_of, total_supply and allowance checks.

fn contract_hash(&self) -> ContractHash {

fn query_contract<T: CLTyped + FromBytes>(&self, name: &str) -> Option<T> {
    match self
        .query(self.ali, &[CONTRACT_KEY_NAME.to_string(), name.to_string()])
        Err(_) => None,
        Ok(maybe_value) => {
            let value = maybe_value
                .unwrap_or_else(|_| panic!("{} is not expected type.", name));

Helper Functions

The following helper functions are defined to query the named keys in the contract.

The token_name function returns the name of the token:

pub fn token_name(&self) -> String {

The token_symbol function returns the token symbol:

pub fn token_symbol(&self) -> String {

The token_decimals function returns the number of decimal places for the token:

pub fn token_decimals(&self) -> u8 {

Invoking contract methods

The following code snippet describes a generic way to call a specific entry point in the contract.

fn call(&mut self, sender: Sender, method: &str, args: RuntimeArgs) {
    let Sender(address) = sender;
    let code = Code::Hash(self.contract_hash().value(), method.to_string());
    let session = SessionBuilder::new(code, args)

The next code sample shows how to invoke one of the methods in the contract. The rest of the methods are available on GitHub.

pub fn balance_of(&self, account: Key) -> Option<U256> {
    let item_key = base64::encode(&account.to_bytes().unwrap());

    let key = Key::Hash(self.contract_hash().value());
    let value = self
        .query_dictionary_item(key, Some(consts::BALANCES_KEY_NAME.to_string()), item_key)


Creating Unit Tests

Now that we have a testing context, we can use it to create unit tests in a file called integration_tests.rs. The unit tests verify the contract code by invoking the functions defined in the test_fixture.rs file.

The example below shows one of the example tests. Visit GitHub to find all the available tests.

// File https://github.com/casper-ecosystem/erc20/blob/master/example/erc20-tests/src/integration_tests.rs

use casper_types::{Key, U256};

use crate::test_fixture::{Sender, TestFixture};

fn should_install() {
    let fixture = TestFixture::install_contract();
    assert_eq!(fixture.token_name(), TestFixture::TOKEN_NAME);
    assert_eq!(fixture.token_symbol(), TestFixture::TOKEN_SYMBOL);
    assert_eq!(fixture.token_decimals(), TestFixture::TOKEN_DECIMALS);

Running the Tests

We have configured the lib.rs file to run the example integration tests via the make test command:

mod lib_integration_tests;

To run the tests, navigate to the parent erc20 directory and run the make test command:

make test

This example uses bash. If you are using a Rust IDE, you need to configure it to run the tests.