Does anyone know how to fork a chain at a particular block on Brownie?

Question

I am trying to fork the ethereum mainnet at a past block number on brownie. But I dont see any way to do that in the brownie docs .

Answer 1

To fork a blockchain with brownie, you actually just need to create a network that uses a forked blockchain or uses a forking command.

Option 1: The built-in forked chain

Brownie actually comes packed with a forked feature:

$ brownie networks list
Brownie - Python development framework for Ethereum

The following networks are declared:

Ethereum
  ├─Mainnet (Infura): mainnet
  ├─Ropsten (Infura): ropsten
  ├─Rinkeby (Infura): rinkeby
  ├─Goerli (Infura): goerli
  └─Kovan (Infura): kovan

Ethereum Classic
  ├─Mainnet: etc
  └─Kotti: kotti

Development
  ├─Ganache-CLI: development
  └─Ganache-CLI (Mainnet Fork): mainnet-fork <- This is the network!!

It uses your Infura key to fork mainnet to a forked ganache instance.

Note: I have found more success using Alchemy

Option 2: Add a new network that uses a forking command (recommended)

You can optionally add a network that automatically forks a blockchain of your choice.

Run the following command:

brownie networks add development chain-name-fork cmd=ganache-cli host=http://127.0.0.1 fork=RPC_URL accounts=10 mnemonic=brownie port=8545

If you want to pin a specific block, add @<block_number>

brownie networks add development chain-name-fork cmd=ganache-cli host=http://127.0.0.1 fork=RPC_URL@123456 accounts=10 mnemonic=brownie port=8545

This will add a network called chain-name-fork that will run on http://127.0.0.1. You just need to pass it an RPC_URL for the fork parameter, of which blockchain it should fork.

*Note: If using ganache, you can use any ganache-cli commands.

You can also do this with hardhat if you choose. The block pin parameter is --fork-block-number 11095000

Option 3: You fork a blockchain yourself, and just point to that

Maybe, you want to run a local hardhat node like so:

npx hardhat node --fork https://eth-mainnet.alchemyapi.io/v2/<key> --fork-block-number 11095000

You'll now have a local blockchain running. You can then add it with something like:

brownie networks add Ethereum ganache host=http://localhost:8545 chainid=1337