If I were going to fix this problem (which I may well do soon -- I have to take another pass through this material when writing the book), I would actually define the __getitem__() function instead, as that controls the behavior of the  operator.
This all comes down to separation of concerns. It shouldn't be the responsibility of __next__() to mutate the internal data to obscure information. It's only job should be to determine whether it exposes that information, and how.
Of course, in all honesty, there's nothing preventing a direct call to agents._roster (Python has no private variables). If we were going to obfuscate or remove classified data, that should really occur on the add_agent() function.
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