I didn't have previous exposure to that exact syntax either, but:
- Python and plenty other languages have assignment variations with an operator like
+ |in plenty of languages means logical OR- possibly bitwise, but that seems unlikely in Python
- Dicts are sorta complex siblings of sets
- In terms of sets, OR is equivalent to a union
Hence, a |=b should have the same meaning as a = a | b or a = a.union(b)
and then the quiz is about basically implementation details and what the actual semantics winds up being in a language like python, where stuff is mutable by default and references are implicit (even though the zen of python states a preference for being explicit)