IndexError raised when passing an element of brownie.accounts as the exclude argument to brownie.test.strategy

Environment information

• brownie Version: 1.12.3, 1.12.2, 1.12.1
• ganache-cli Version: 6.12.1
• solc Version: 0.8.0+commit.c7dfd78e.Linux.g++
• Python Version: 3.8.3, 3.9.1
• OS: linux

What was wrong?

Command: brownie test Code that caused failure:

from brownie import accounts
from brownie.test import given, strategy

@given(
    amount=strategy("uint256", max_value="1000 ether"),
    to=strategy("address", exclude=accounts[0]),
)
def test_transfer_success_returns_true(token, accounts, amount, to):
    assert token.transfer(to, amount, {"from": accounts[0]}) == True

Error output:

$ brownie test tests/test_transfer.py
Brownie v1.12.3 - Python development framework for Ethereum

=============================================================================================== test session starts ===============================================================================================
platform linux -- Python 3.8.3, pytest-6.0.1, py-1.10.0, pluggy-0.13.1
rootdir: /home/skelletor/projects/CloudToken
plugins: eth-brownie-1.12.3, hypothesis-5.41.3, forked-1.3.0, web3-5.11.1, xdist-1.34.0
collected 0 items / 1 error                                                                                                                                                                                       

===================================================================================================== ERRORS ======================================================================================================
_____________________________________________________________________________________ ERROR collecting tests/test_transfer.py _____________________________________________________________________________________
tests/test_transfer.py:8: in <module>
    to=strategy("address", exclude=accounts[0]),
E   IndexError: list index out of range
============================================================================================= short test summary info =============================================================================================
FAILED tests/test_transfer.py - IndexError: list index out of range
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
================================================================================================ 1 error in 0.15s =================================================================================================

Extra Information

• I was following @iamdefinitelyahuman’s In-Depth Guide to Testing Ethereum Smart Contracts Part Six, which was published on July 23, 2020. Since the test in the guide and mine are essentially the same, and assuming that the code in the guide worked when the content was published, then the bug was introduced between July 23, 2020 and now.
• I’ve successfully recreated the error above with the v1.12.x releases.
• Using slice notation (accounts[:1]), the IndexError doesn’t get raised, however, the element accounts[0] is included in the sampling. This makes sense, because a slice of an empty list, will return an empty list, which is essentially excluding nothing.

[iamdefinitelyahuman]

Took a look at this… As you said, the problem is that test collection happens prior to connecting to the network, so accounts is empty.

Connecting to the network is handled in pytest_collection_finish, in brownie/test/runner.py, line 258. This is a deliberate design choice for 2 reasons:

1. If no tests were collected, or there is an error during collection, we don’t have to wait for ganache-cli to launch and be killed.
2. It allows for individual test suites to implement their own hook point and connect to a different network (example)

There is no easy solution I can see here unfortunately, as the call to strategy happens immediatly upon importing the test module. A hacky solution might be to allow Accounts to return a sort-of proxy “promise” object that represents the eventual Account that will exist at index 0 once the network has connect. The first attempt to inspect this object would then mutate it into the actual account object, or raise if the index is out of bounds.

[sabotagebeats]

I was hoping to find a solution which would wrap this for the user so that they don’t have to worry about it, however it seems that it’s functioning as designed and so this won’t be feasible? If this is the case, alternatively could we just modify the documentation for the address strategy in order to notate that it requires loading the accounts object first?