Guide: Unit Tests on Forked Mainnet (Lesson 10)

[ZeroEkkusu]

Gm, fellow devs,

In Lesson 10, we have unit tests for our Python scripts.

If you tried making your own, though, some would fail because the state of the chain would change with each test.

Here's an example:

Test 1:
> Mint 1 WETH
> Assert you have 1 WETH
✔️ PASSED

Test 2:
> Mint 1 WETH
> Spend 1 WETH
> Assert you have 0 WETH
❌ FAILED - you have 1 WETH, not 0 WETH

That's because Test 1 changed your WETH balance to 1 and you started Test 2 with that 1 WETH. In other words, the chain did not reset before running Test 2.

Here's how to do unit tests in the mainnet-fork-dev environment:

• Make the conftest.py file in the tests directory
• Add the following

import pytest
from scripts.helpful_scripts import LOCAL_BLOCKCHAIN_ENVIRONMENTS
from brownie import network, chain


@pytest.fixture(scope="module")
def test_only_locally():
    if network.show_active() not in LOCAL_BLOCKCHAIN_ENVIRONMENTS:
        pytest.skip()
    else:
        chain.reset()


@pytest.fixture(scope="module")
def dont_test_locally():
    if network.show_active() in LOCAL_BLOCKCHAIN_ENVIRONMENTS:
        pytest.skip()

• Now, pass the fixtures to your tests - some examples:

def test_mint_weth(test_only_locally):
    # ...

def test_basic_integration(dont_test_locally):
    # ...

• Test!

brownie test --network mainnet-fork-dev

brownie test --network kovan

Explanation

• A @pytest.fixture is something that will run prior to (and/or after) the test you pass it to
• scope="module" means that, if passed to a test, it will run once per file (module) - I keep my unit tests in separate files inside the tests/unit directory. Use scope="function" if you keep them all in a single file
• chain.reset() takes a snapshot before running the test and reverts to it when the test is done - ensuring the initial conditions are always the same
• As you can see, we have our network checks, as well, so, no need to do them in our tests

Btw, you can do the "Arrange" part of your test like this, too.
More info here.

HNY! 🥳

[cromewar]

This is an amazing contribution for the repository, thanks for this!!

[PatrickAlphaC]

Awesome work here!! Love it :D