On Sun, Nov 4, 2012 at 12:14 PM, Oscar Benjamin <oscar.j.benja...@gmail.com> wrote: > On 3 November 2012 22:50, Chris Angelico <ros...@gmail.com> wrote: >> This one I haven't checked the source for, but ISTR discussions on >> this list about comparison of two unequal interned strings not being >> optimized, so they'll end up being compared char-for-char. Using 'is' >> guarantees that the check stops with identity. This may or may not be >> significant, and as you say, defending against an uninterned string >> slipping through is potentially critical. > > The source is here (and it shows what you suggest): > http://hg.python.org/cpython/file/6c639a1ff53d/Objects/unicodeobject.c#l6128 > > Comparing strings char for char is really not that big a deal though. > This has been discussed before: you don't need to compare very many > characters to conclude that strings are unequal (if I remember > correctly you were part of that discussion).
Yes, and a quite wide-ranging discussion it was too! What color did we end up whitewashing that bikeshed? *whistles innocently* > I can imagine cases where I might consider using intern on lots of > strings to speed up comparisons but I would have to be involved in > some seriously heavy and obscure string processing problem before I > considered using 'is' to compare those interned strings. That is > confusing to anyone who reads the code, prone to bugs and unlikely to > achieve the desired outcome of speeding things up (noticeably). Good point. It's still true that 'is' will be faster, it's just not worth it. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
