Python: __bool__ does not raise TypeError by default

[yoff]

This PR just adds a test to highlight the problem. I will later add a commit to it to also fix the problem.

[RasmusWL]

I think a little more context for why __bool__ is a special case is very helpful. I will gladly admit I didn't know the default behavior of __bool__, and had to read the FP report to figure out why we needed this change. I have given a suggestion for an explanatory comment, but feel free to formulate it differently 😉

A general approach I find very useful when fixing FPs is to use a 2-step process:

1. add FP triggering code to a test-file and update .expected results of the qltest
2. fix the FP in the query and (again) update the .expected results of the qltest

In this way it's very easy to see what the FP-alert was, just by reviewing the commits one-by-one (although the .expected file doesn't change when viewing the diff of all commits). No need to change this PR, but hopefully this can be a useful method to keep in mind for the future 😊

[yoff]

Thanks, just to make sure I understand: Between step 1 and 2, the .expected-file does not actually reflect what we know of the world? That feels a bit wrong :-) Is it not possible to see the intermediate failings?

[yoff]

Ah, Taus has explained this in more detail in my onboarding issue.

[RasmusWL]

What would happen if we now raise a wrong exception for __bool__ ? Before, we would get an alert saying we didn't need to implement it at all (which was wrong), but from reading the code, I don't see how the following code would give any alert.

class Foo(object):
    def __bool__(self):
        raise ZeroDivisionError()

[yoff]

That is a good point, I suppose that should be considered part of solving the problem :-)

[yoff]

Ah, now I see why the merge was needed. It is because I committed your suggested change. I should've just pulled right after that.

[yoff]

redo failed

[yoff]

Check failed due to incorrect line numbers. I must have not run the tests after changing comments or something.

[RasmusWL]

Nice, very good job 👍