Testing Smart Contracts

ℹ️ Info To use the library functions designed for testing smart contracts, you need to add snforge_std package as a dependency in your Scarb.toml using appropriate release tag.

[dev-dependencies]
snforge_std = { git = "https://github.com/foundry-rs/starknet-foundry.git", tag = "v0.12.0" }

Using unit testing as much as possible is a good practice, as it makes your test suites run faster. However, when writing smart contracts, you often want to test their interactions with the blockchain state and with other contracts.

The Test Contract

In this tutorial we will be using this Starknet contract

#[starknet::interface]
trait IHelloStarknet<TContractState> {
    fn increase_balance(ref self: TContractState, amount: felt252);
    fn get_balance(self: @TContractState) -> felt252;
}

#[starknet::contract]
mod HelloStarknet {
    #[storage]
    struct Storage {
        balance: felt252,
    }

    #[abi(embed_v0)]
    impl HelloStarknetImpl of super::IHelloStarknet<ContractState> {
        // Increases the balance by the given amount.
        fn increase_balance(ref self: ContractState, amount: felt252) {
            self.balance.write(self.balance.read() + amount);
        }

        // Gets the balance.
        fn get_balance(self: @ContractState) -> felt252 {
            self.balance.read()
        }
    }
}

Note that the name after mod will be used as the contract name for testing purposes.

Writing Tests

Let's write a test that will deploy the HelloStarknet contract and call some functions.

use snforge_std::{ declare, ContractClassTrait };

#[test]
fn call_and_invoke() {
    // First declare and deploy a contract
    let contract = declare("HelloStarknet").unwrap();
    // Alternatively we could use `deploy_syscall` here
    let (contract_address, _) = contract.deploy(@array![]).unwrap();

    // Create a Dispatcher object that will allow interacting with the deployed contract
    let dispatcher = IHelloStarknetDispatcher { contract_address };

    // Call a view function of the contract
    let balance = dispatcher.get_balance();
    assert(balance == 0, 'balance == 0');

    // Call a function of the contract
    // Here we mutate the state of the storage
    dispatcher.increase_balance(100);

    // Check that transaction took effect
    let balance = dispatcher.get_balance();
    assert(balance == 100, 'balance == 100');
}

📝 Note

Notice that the arguments to the contract's constructor (the deploy's calldata argument) need to be serialized with Serde.

HelloStarknet contract has no constructor, so the calldata remains empty in the example above.

$ snforge test
Collected 1 test(s) from using_dispatchers package
Running 0 test(s) from src/
Running 1 test(s) from tests/
[PASS] tests::call_and_invoke
Tests: 1 passed, 0 failed, 0 skipped, 0 ignored, 0 filtered out

Handling Errors

Sometimes we want to test contracts functions that can panic, like testing that function that verifies caller address panics on invalid address. For that purpose Starknet also provides a SafeDispatcher, that returns a Result instead of panicking.

First, let's add a new, panicking function to our contract.

#[starknet::interface]
trait IHelloStarknet<TContractState> {
    // ...
    fn do_a_panic(self: @TContractState);
    fn do_a_string_panic(self: @TContractState);
}

#[starknet::contract]
mod HelloStarknet {
    use array::ArrayTrait;

    // ...

    #[abi(embed_v0)]
    impl HelloStarknetImpl of super::IHelloStarknet<ContractState> {
        // ...

        // Panics
        fn do_a_panic(self: @ContractState) {
            let mut arr = ArrayTrait::new();
            arr.append('PANIC');
            arr.append('DAYTAH');
            panic(arr);
        }

        fn do_a_string_panic(self: @ContractState) {
            assert!(false, "This is panicking with a string, which can be longer than 31 characters");
        }
    }
}

If we called this function in a test, it would result in a failure.

#[test]
#[feature("safe_dispatcher")]
fn failing() {
    // ...

    let (contract_address, _) = contract.deploy(@calldata).unwrap();
    let dispatcher = IHelloStarknetDispatcher { contract_address };

    dispatcher.do_a_panic();
}
$ snforge test
Collected 1 test(s) from package_name package
Running 0 test(s) from src/
Running 1 test(s) from tests/
[FAIL] tests::failing

Failure data:
    (0x50414e4943 ('PANIC'), 0x444159544148 ('DAYTAH'))

Tests: 0 passed, 1 failed, 0 skipped, 0 ignored, 0 filtered out

Failures:
    tests::failing

SafeDispatcher

Using SafeDispatcher we can test that the function in fact panics with an expected message.

#[test]
#[feature("safe_dispatcher")]
fn handling_errors() {
    // ...

    let (contract_address, _) = contract.deploy(@calldata).unwrap();
    let safe_dispatcher = IHelloStarknetSafeDispatcher { contract_address };


    match safe_dispatcher.do_a_panic() {
        Result::Ok(_) => panic_with_felt252('shouldve panicked'),
        Result::Err(panic_data) => {
            assert(*panic_data.at(0) == 'PANIC', *panic_data.at(0));
            assert(*panic_data.at(1) == 'DAYTAH', *panic_data.at(1));
        }
    };
}

Now the test passes as expected.

$ snforge test
Collected 1 test(s) from package_name package
Running 0 test(s) from src/
Running 1 test(s) from tests/
[PASS] tests::handling_errors
Tests: 1 passed, 0 failed, 0 skipped, 0 ignored, 0 filtered out

Similarly, you can handle the panics which use ByteArray as an argument (like an assert! or panic! macro)

// Necessary utility function import
use snforge_std::byte_array::try_deserialize_bytearray_error;

// ...
#[test]
#[feature("safe_dispatcher")]
fn handling_string_errors() {
    // ...
    let (contract_address, _) = contract.deploy(@calldata).unwrap();
    let safe_dispatcher = IHelloStarknetSafeDispatcher { contract_address };
    
    match safe_dispatcher.do_a_panic_with_bytearray() {
        Result::Ok(_) => panic_with_felt252('shouldve panicked'),
        Result::Err(panic_data) => {
            let str_err = try_deserialize_bytearray_error(panic_data.span()).expect('wrong format');
            assert(
                str_err == "This is a very long\n and multiline message that is certain to fill the buffer", 
                'wrong string received'
            );
        }
    };
}

You also could skip the de-serialization of the panic_data, and not use try_deserialize_bytearray_error, but this way you can actually use assertions on the ByteArray that was used to panic.

📝 Note

To operate with SafeDispatcher it's required to annotage its usage with #[feature("safe_dispatcher")].

There are 3 options:

  • module-level declaration
    #[feature("safe_dispatcher")]
    mod my_module;    
  • function-level declaration
    #[feature("safe_dispatcher")]
    fn my_function() { ... }    
  • directly before the usage
    #[feature("safe_dispatcher")]
    let result = safe_dispatcher.some_function();

Expecting Test Failure

Sometimes the test code failing can be a desired behavior. Instead of manually handling it, you can simply mark your test as #[should_panic(...)]. See here for more details.