Comment by kbd

> Interestingly enough some languages actually do the opposite.

Of course you can represent sets as dictionaries with empty values (ask anyone who programmed Perl). You're supporting my point that dicts logically subclass sets, because they can represent sets where the values can be other things as well.

You're also getting at what the behavior should be if you union a dictionary and a set. Hypothetical Python:

    >>> s = {'a','b','c'}
    >>> d = {'d': 'D'}
    >>> d | s
    >>> {'d': 'D', 'a': None, 'b': None, 'c': None}