🐍 Testing Vyper Contracts with Foundry (t.sol)

This repo demonstrates a streamlined workflow for testing Vyper smart contracts using Foundry’s Solidity test framework (t.sol).
By combining Moccasin for Vyper scaffolding and Foundry for testing, you get the best of both worlds — clean Vyper code with powerful, fast Solidity-based tests.

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Why This Matters

Testing Vyper contracts directly in Foundry:

• ✅ Avoids writing separate Python test suites.
• ✅ Lets you use Foundry’s cheatcodes (deployCode, prank, deal, etc.).
• ✅ Integrates seamlessly with Solidity tooling, coverage, and CI pipelines.
• ✅ Speeds up iteration — deploy and test in-memory without spinning up external chains.

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🛠 Process Overview

1. Initialize a Moccasin template

   mox init 

2. Force initialize a Foundry template in the same directory

   forge init --force 

3. Write your Vyper contracts

   • Place .vy files in src/ (e.g., src/Counter.vy).
   • Keep them clean and idiomatic Vyper.

4. Write Solidity interfaces for your Vyper contracts

   • Example: src/ICounter.sol (This matches the ABI of your Vyper contract so Solidity tests can call it.)

5. (Optional) Write a deploy script

   • Useful for deploying to a live chain, but not required for Foundry tests.

6. Write Foundry tests in the test folder

   • Use deployCode("Counter.vy") to deploy your Vyper contract inside the test environment.

   for instance:

   contract CounterTest is Test {
       ICounter public counter;
   
       function setUp() public {
           counter = ICounter(deployCode("Counter.vy"));
       }
   
       function test_SetNumber() public {
           counter.set_number(42);
           assertEq(counter.number(), 42);
       }
   }

Running Tests

forge test -vvv

Tips

Keep complex types (DynArray, HashMap) private in Vyper and write explicit getters to avoid ABI mismatches