Revert Reasons

A contract may revert during the runtime via the REVERT opcode. During execution, the revert does exactly the same thing independent on the error source. However, for a developer, there are different reasons to get a revert. Each of them may be used with boa.reverts to test the contract behavior.

Compiler Revert Reasons

These happen when the compiler generates the error message. For example: - Range errors - Overflows - Division by zero - Re-entrancy locks

Things like syntax errors will not be caught during the runtime, but the contract will fail to compile on the first place.

User Revert Reasons

These happen when the user calls raise or assert fails in the contract. The user may provide a reason for the revert, which will be shown to the end user.

Vyper

@external
def foo(x: uint256):
    assert x > 0, "x must be greater than 0"

Note that this may happen directly on the contract being called, or any external contract that the contract interacts with.

Dev Revert Reasons

Developer reverts are also raised by assert statements in the code. However, by adding a # dev: <reason> comment after the assert call, Titanoboa is able to verify the reason and provide a more detailed error message.

Vyper

@external
def foo(x: uint256):
    assert x > 0 # dev: "x must be greater than 0"

These reasons are completely offchain and useful when the contract storage is limited (EIP 170).

Traditional revert strings cost gas and bytecode when deploying. When a revert condition is triggered, extra gas will also be incurred. Strings are relatively heavy - each character consumes gas.

However, when using VyperContract, Titanoboa is able to track the line where the revert happened. If it finds a # dev: <reason> comment, it will provide a more detailed error message to the developer.

This is particularly useful when testing contracts with boa.reverts, for example:

Python

with boa.reverts(dev="x must be greater than 0"):
    contract.foo(0)