Add a helper to test traces

We should add a new method to pytest plugin. We need to test traces.

Here’s the usecase we are trying to cover. Imagine, there are multiple places where Failure can happen with the same result. For example:

def first() -> ResultE[int]:
    return Failure(KeyError('a'))

def second() -> ResultE[int]:
    return Failure(KeyError('a'))

And we need to test that the reason why our app failed is first() call, not second() Matching by value is not good enough here, since values are identical. But, we can use traces!

def test_failure(returns):
    assert returns.has_trace(my_function(), 'some trace object with `first()` call')

And it will pass for Failure created by first() and fail for Failure created by second()

Related to #409 CC @thepabloaguilar

[thepabloaguilar]

I’m implementing and I’ve decided to change a bit the API. It will be like pytest.raises:

def test_something(returns) -> None:
    with returns.has_trace(Success, function_foo):
        function_bar()

[sobolevn]

LGTM! 👍

Thanks a lot!