Stefan Behnel, 09.10.2010 08:21:
> Nikolaus Rath, 09.10.2010 03:15:
>> It seems that Cython translates the line
>>
>> if ent.d_name in (b'.', b'..'):
>>
>> as well as
>>
>> if ent.d_name == b'.' or ent.d_name == '..':
>>
>> into pointer comparison rather than a strcmp():
>>
>> if (!(__pyx_v_ent->d_name == __pyx_k_1)) {
>> __pyx_t_3 = (__pyx_v_ent->d_name == __pyx_k_2);
>> } else {
>> __pyx_t_3 = (__pyx_v_ent->d_name == __pyx_k_1);
>> }
>> if (__pyx_t_3) {
>>
>> (__pyx_k_2 and __pyx_k_1 are "." and "00" respectively).
>
> That's not explicitly intended but a side effect of a couple of
> implementation details and optimisations in Cython. The main problem
> here is that Cython's byte strings start off as char* values, and "=="
> on pointers is the same as "is", i.e. it compares the pointers.
>
> I think it's ok to consider this specific case a bug. If a user wants
> pointer comparison, "is" is the most explicit operator w.r.t. Python
> semantics. Using strcmp() for the "==" operator on char* values makes
> sense to me. We special case char* all over the place, so this is just
> another exception.
>
> So, for char* values, "s1 == s2" should translate into
>
> (s1 == s2) || (
> (s1 != NULL) && (s2 != NULL) && (strcmp(s1, s2) == 0))
>
> Note that this allows both pointers to be NULL in the success case. I
> think it makes sense to include this.
>
> This change certainly introduces new corner cases, though, as "ptr ==
> ptr" means "ptr is ptr" for everything else. So this will surprise at
> least some users and also break existing code. But I think it's at least
> mostly obvious to the average Python developer what the right fix is.
>
> One step further: Would it be worth making "is" the correct operator for
> pointers in the long run? I.e. discourage the use of "==" for pointers
> and eventually make it an error? Personally, I often feel an itch in the
> back of my head when I write "ptr == ptr" and I know that what I
> actually mean is "ptr is ptr". I still write it most of the time because
> I find "==" more quickly readable than the chain of words involving
> "is", which involves text reading instead of just operator spotting.
http://trac.cython.org/cython_trac/ticket/582
Stefan
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