[pypy-dev] Object identity and dict strategies

[Maciej Fijalkowski]

On Fri, Jul 8, 2011 at 4:14 PM, Amaury Forgeot d'Arc <amauryfa at gmail.com> wrote:
> 2011/7/8 Cesare Di Mauro <cesare.di.mauro at gmail.com>:
>> I fully agree. It's not an issue, but an implementation-specific detail
>> which programmers don't have to assume always true.
>>
>> CPython can be compiled without "smallints" (-5..256, if I remember
>> correctly) caching. There's a #DEFINE that can be disabled, so EVERY int (or
>> long) will be allocated, so using the is operator will return False most of
>> the time (unless you are just copied exactly the same object).
>>
>> The same applies for 1 character strings, which are USUALLY cached by
>> CPython.
>
> But the problem here is not object cache, but preservation of object identity,
> which is quite different.
> Python containers are supposed to keep the objects you put inside:

[citation needed] array.array does not for one

>
> myList.append(x)
> assert myList(-1) is x
>
> myDict[x] = 1
> for key in myDict:
>    if key is x:
>        ...
>

also dict doesn't work if you overwrite the key:

d = {1003: None}
x = 1003
d[x] = None
d.keys()[0] is x