On Wed, 20 May 2015 03:07:03 +1000, Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote:
>Yes, a slice can be expensive, if you have (say) a ten billion element list, >and take a slice list[1:]. Since nothing seems to surprise you and you seem so adamant on calling anyone being surprised by it, maybe I will surprise you if you actually run the code I posted on the OP and witness for yourself that even on a 50 element list will take 3 seconds to execute on an intel i5. >But, really... if you're serious about dealing with huge arrays of data, you >want something like numpy, not Python lists. You don't need huge. On any algorithm where slices are being used you will have to account for the linear complexity. You seem to be dismissive of this fact, but it can have tremendous performance implications, since it can turn a linear algorithm into a quadratic one just like that. Certainly there are alternatives and iterating through the list is a much better method. There's no arguing against that. But if you are paying attention to how Python is being taught, [1:] is a little everywhere on the web announced as a cool trick showcasing Python strengths. Well, it turns out it is actually a bad practice performance-wise under many cases and the alternatives should be the ones being showcased. > So *in practice* the lack of views for lists is a minor nuisance, if that. Your opinion was noted. > Having list slices return copies rather than views avoids more problems than > it causes. Well, I personally wouldn't want to see slices do anything other than what they are doing now. I think that is the point. Backwards compatibilioty being just one of the problems. But no one is arguing for that. Instead, it was said that it would be interesting if Python offered views. >On the other hand, if Python implementations would implement slices with >copy-on-write, that would give us the best of both worlds! You are a confusing person. You after all agree with what is being said, but spend all your time talking against it. -- https://mail.python.org/mailman/listinfo/python-list
