Allow specify block to fork at for "mainnet-fork"

Ganache supports specifying the block to fork at: -f or --fork: Fork from another currently running Ethereum client at a given block. Input should be the HTTP location and port of the other client, e.g. http://localhost:8545. You can optionally specify the block to fork from using an @ sign: http://localhost:8545@1599200.

Would it be possible to surface this as a config or CLI option in brownie?

[gosuto-inzasheru]

i like that work-around @0xalfalfa! i was thinking something similar but with some bash magic and run it through brownie run.

a proper implementation doesn’t seem far away though, after finding out that ganache-cli already has the parameter --fork_block_number: https://github.com/trufflesuite/ganache/blob/master/README.md#options.

appears to me it only needs to be supported by the cmd-settings part of the app that passes args to ganache? eg brownie run --network mainnet-fork --fork_block_number 13131313.

dont know if i am the one to tackle this, but if anyone could give pointers on where to look in the code in order to prep a pr that would be cool 😃 initial thought is to start by adding the cli flag here: https://github.com/eth-brownie/brownie/blob/master/brownie/network/rpc/ganache.py

[0xalfalfa]

Here’s my workaround:

• You need an archive node to go back more than 128 blocks. You can get one with the with Alchemy free tier.
• In brownie-config.yaml set the fork up like this (adapt accordingly if you have a local archive node):

  mainnet-fork:
    cmd_settings:
        fork: https://eth-mainnet.alchemyapi.io/v2/$ALCHEMY_KEY$FORK_BLOCK_NUMBER

• In your .env file set ALCHEMY_KEY=<ALCHECHMY_KEY>
• If you always want to go back to the same block, also set e.g. FORK_BLOCK_NUMBER=@12710000 in the .env file.
• If you want to go to a specific block in a python script, e.g. a test case, set the FORK_BLOCK_NUMBER environment variable in the first line of your python code:

import os
os.environ["FORK_BLOCK_NUMBER"] = "@127110000"

This will only work when starting your python script yourself or through your VSCode debugger or such. If you start with brownie run or brownie test, you have to set all env variables before starting brownie.