I see you solved your issue, however there was one thing I want to point out.
The Python error messages actually tell you a lot about what's going wrong, to the point where I could figure out some of the issues completely statically. One example is where it's telling you that the function __add__ is not defined. We can see in your code that your indents are all over the place, putting the __add__ function within the __ne__ function, so it's invisible (nonexistent) from the outside.

Also here, it directly tells you your indent is off and you seem just not to read these error messages. They exist for a reason.

I think that Jimmy Song here wanted to make readers implement everything in a TDD way, which is a very good idea and teaches learners a lot about the language itself and how to develop cleanly. You make use of the test framework by writing tests beforehand according to spec / concept, then develop until your tests are all 'green'.

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Also, it's actually a rule here not to write consecutive posts, but merging them instead, by the way.