On Fri, 25 Mar 2011 13:56:23 +0100, Antoon Pardon wrote: > Look we are provided with the cmp_to_key function. If something doesn't > work with that function or performs realy bad with that function, it > means either the function has a bug in it or something the function > relies on, has a bug in it. In either case you just fix the bug.
No, it means that the trade-offs made by cmp_to_key, or by sorting with a key function, do not interact well with what you are trying to do. An analogy: Python lists are contiguous arrays of pointers. When the array is full, Python automatically doubles the size of the list. This is a good trade-off of space (memory) for time, because it means that list access is O(1), searching is O(N), and appending is O(1) on average. At worst, you waste 50% of the memory used, which is pretty good. But if you're programming for a device with 8 K of memory (if such a thing even exists these days!), this would be a *terrible* strategy. (Python wouldn't even fit in 8K, but never mind that...) In this case, memory is more precious than programmer convenience, or even execution speed. It's not enough to hand-wave and say "Python lists have a bug that needs fixing", because that's simply not the case. It's that the design of Python lists doesn't suit the use-case, and if you try it, it will perform badly or not at all: by design, Python lists assume memory will be plentiful and time will be short. That is exactly the same trade-off key-based sorting makes: it uses more memory to speed up sorting. This is a good strategy for Python, because Python already requires lots of memory (so no major loss there), and is a rather slow language (so speed-ups help). So the question is, being completely general, as there any *real-world* (not artificial, made-up) uses where sorting with a comparison function works but a key function doesn't? I'm not limiting you to cases where you have a shortage of memory but lots of time available. There may be other design choices where key-based sorting sucks. We already know that some people just prefer the look and feel of writing and using cmp functions. Is there anything else? -- Steven -- http://mail.python.org/mailman/listinfo/python-list
