[Feature] Use Python assert in smart contract

[hal0x2328]

It would be neat if we could replace something like:

if not CheckWitness(Owner):
    print("Unauthorized user")
    return False

with:

assert CheckWitness(Owner), "Unauthorized user"

I'm moving to just implement this in the following way in my smart contracts:

OnError = RegisterAction('error', 'message')
...
sc_assert(CheckWitness(Owner), "Unauthorized user")

def sc_assert(condition, msg):
    if not condition:
        print(msg) # for neo-python console
        OnError(msg) # for neo-cli ApplicationLog
        throw_if_null(0)

but it would be cool if it were just natively supported with the existing assert keyword in Python.

(I noticed that the neo-cli implementation ofgetapplicationlog returns the notification events that triggered before the contract faulted, so the sc_assert function I wrote takes advantage of that - I see this as being a huge help for people trying to debug contract failures after-the-fact. So if this suggestion were to be implemented as a built-in it would be nice to replicate that feature too.)

[ixje]

I have not looked at what getapplicationlog does, but notification events that triggered before the contract faulted can be logged/displayed with neo-python via config sc-debug-notify. Does that help?

sc-debug-notify - toggle printing smart contract Notify events on execution failure

[hal0x2328]

notification events that triggered before the contract faulted can be logged/displayed with neo-python via config sc-debug-notify. Does that help?

Yes, that could eliminate the need for the extra print statement.

[hal0x2328]

One other issue I am having with my bespoke assert implementation is that it appears to be impossible to have a void return type for a function. So unless I use something like

ret = sc_assert(something_is_true(), "Not true!")

I end up with an extra value on the evaluation stack at the end of execution, which is undesirable. But adding the unused variable assignment is also kind of inelegant.

[hal0x2328]

I think I found the solution and it doesn't require any changes to neo-boa, just a little knowledge of Python internals:

OnError = RegisterAction('error', 'message')

def Main(operation, args):
    assert args[0] == True, 'Got false argument!'
    return True

def AssertionError(msg):
    OnError(msg)
    throw_if_null(0)