Comparing strings from the back?

[Chris Angelico]

On Tue, Sep 11, 2012 at 12:06 AM, Oscar Benjamin
<oscar.j.benjamin at gmail.com> wrote:
> On 2012-09-10, Steven D'Aprano <steve+comp.lang.python at pearwood.info> wrote:
>> What interning buys you is that "s == t" is an O(1) pointer compare if
>> they are equal. But if s and t differ in the last character, __eq__ will
>> still inspect every character. There is no way to tell Python "all
>> strings are interned, if s is not t then s != t as well".
>>
>
> I thought that if *both* strings were interned then a pointer comparison could
> decide if they were unequal without needing to check the characters.
>
> Have I misunderstood how intern() works?

In a language where _all_ strings are guaranteed to be interned (such
as Lua, I think), you do indeed gain this. Pointer inequality implies
string inequality. But when interning is optional (as in Python), you
cannot depend on that, unless there's some way of recognizing interned
strings. Of course, that may indeed be the case; a simple bit flag
"this string has been interned" would suffice, and if both strings are
interned AND their pointers differ, THEN you can be sure the strings
differ.

I have no idea whether or not CPython version X.Y.Z does this. The
value of such an optimization really depends on how likely strings are
to be interned; for instance, if the compiler automatically interns
all the names of builtins, this could be quite beneficial. Otherwise,
probably not; most Python scripts don't bother interning anything.

ChrisA