[Python-3000] Immutable bytes type and dbm modules

["Martin v. Löwis"]

>>> The most efficient representation of immutable bytes is quite different
>>> from the most efficient representation of mutable bytes.
>>
>> In what way?
> 
> Well, in some runtime environments (I'm not sure about Python), for
> immutables you can combine the object header and the bytes array into a
> single allocation. Further, the header need not contain an explicit
> pointer to the bytes themselves, instead the bytes are obtained by doing
> pointer arithmetic on the header address.

Hmm. That assumes that the mutable bytes type also supports changes to
its length. I see that the Python bytes type does that, but I don't
think it's really necessary - I'm not even sure it's useful.

For a bytes array, you don't need a separate allocation, and it still
can be mutable.

> So in other words, the in-memory layout of the two structs is different
> enough that attempting to combine them into a single struct is kind of
> awkward.

... assuming the mutable bytes type behaves like a Python list, that
is. If it behaved like a Java/C byte[], this issue would not exist.

Regards,
Martin