On Mon, Mar 8, 2010 at 4:21 AM, Steven D'Aprano < st...@remove-this-cybersource.com.au> wrote: > On Sun, 07 Mar 2010 22:31:00 -0800, Raymond Hettinger wrote: >> On Mar 7, 5:46 pm, Steven D'Aprano <st...@remove-this- >> cybersource.com.au> wrote: >>> Given that Counter supports negative counts, it looks to me that the >>> behaviour of __add__ and __sub__ is fundamentally flawed. You should >>> raise a bug report (feature enhancement) on the bug tracker. >> >> It isn't a bug. I designed it that way. There were several possible >> design choices, each benefitting different use cases. <snip> >> Instead the choice was to implement the four methods as multiset >> operations. As such, they need to correspond to regular set operations. > > Personally, I think the behaviour of + and - would be far less surprising > if the class was called Multiset. Intuitively, one would expect counters > to be limited to ints, and to support negative counts when adding and > subtracting. In hindsight, do you think that Multiset would have been a > better name?
Of course I'm not Raymond, but I'd encourage reading the original announcement thread where that and related issues were discussed: http://mail.python.org/pipermail/python-list/2009-January/1189466.html To summarize the arguments against "Multiset": * the API is primarily dict-like, not set-like ** related to this, len() and .items() don't behave in what is arguably an "intuitive" way for multisets * the API allows for negative counts; sets don't normally allow negative multiplicities The same pretty much also goes for why "Bag" wouldn't be an appropriate name. Finally, it leaves the names available for when a proper Bag/Multiset is added. :) Cheers, Chris -- http://blog.rebertia.com
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