I've had use cases for this, and written a class to do it (using Stephen's two dict approach. I'd be really surprised if there wasn't one on PyPi --the trick is to know what to call it to search for it.
Now that I think about it -- my use case was many-to-many, not one to one -- so not quite the same -- each value was a list associated with a given key -- and vice versa. I think it would be a bad idea to overload the built in dict with anything like this. *maybe* a new class for the collections module, if you can find a good well respected implementation, and folks can agree on what's wanted (performance characteristics, API, etc..) -- that's a pretty tall order though. If O(log N) is good enough and bijectivity is guaranteed by some other > mechanism, a bisection search on d.items() with key=lambda x: x[1] > does the trick. > You'd also have to keep it sorted by value. So now you are guaranteeing bijectivity and keeping it sorted -- I'd just use two dicts :-) Though then the values would have to be hashable, so there's that. -CHB > > If you know the dict is supposed to be one-to-one and the keys and > values are disjoint sets, as in your example, just > > def bijective_add(d, k, v): > if k in d and d[k] != v: > raise BijectiveDictValueChangeError > d[k] = v > d[v] = k > > gives O(1) both ways. Maybe you need some kind of check to be sure > you're retrieving the right type for the calling code. > > Otherwise you can pair two dicts in a class and endow it with your > inverse method. I would do the check for bijectivity on addition (as > above) rather than on retrieval, though. > > If you really want to save the space, you can add an additional > hashtable for the values in a C module, but it's not clear to me that > any particular choice for bijectivity checks would be universally > desirable in applications so that seems premature. > > So I think you need to make clear what your goals are since there are > at least four solutions with varying performance characteristics. > I'm agnostic on whether a dict type which guarantees bijectivity would > be a good addition. The mathematician in me wants it, but my > experience says the dict pair is good enough, YAGNI (for values of > "you" == "me", anyway). > > Regards, > Steve > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/5EEE27MQZLFYF3HJMFVR3KINPIZB3HRI/ > Code of Conduct: http://python.org/psf/codeofconduct/ > -- Christopher Barker, PhD (Chris) Python Language Consulting - Teaching - Scientific Software Development - Desktop GUI and Web Development - wxPython, numpy, scipy, Cython
_______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/35XXAKC4CMJYUTNAZ6VJEYGVAJX7RJX2/ Code of Conduct: http://python.org/psf/codeofconduct/
