[PyQt] Access to QSharedMemory with buffer protocol

[Phil Thompson]

On Tue, 02 Nov 2010 13:19:06 +0100, Giovanni Bajo <rasky at develer.com>
wrote:
> On mar, 2010-11-02 at 13:57 +0200, Hans-Peter Jansen wrote:
>> On Tuesday 02 November 2010, 11:26:46 Giovanni Bajo wrote:
>> > On mar, 2010-11-02 at 10:16 +0000, Phil Thompson wrote:
>> > > On Tue, 2 Nov 2010 00:00:06 +0200, "Hans-Peter Jansen"
>> > > <hpj at urpla.net>
>> > >
>> > > wrote:
>> > > > Hi Phil,
>> > > >
>> > > > end of July last year, Alexandre Raczynski asked about accessing
>> > > > QSharedMemory, and you replied, that:
>> > > >
>> > > > mutable_data = buffer(sharedMemory.data())
>> > > >
>> > > > ...should give _write_ access to the memory.
>> > >
>> > > ...bad advice. I didn't realise that you can't create mutable
>> > > buffers from Python. (Although there is no reason why you shouldn't
>> > > be able to.)
>> >
>> > Actually, Python 2.7 has memoryview(); if
>> > memoryview(sharedMemory.data()) worked, it would be possible to use
>> > memoryview's method to mutate the buffer, without having to come up
>> > with a different API for sip.voidptr.
>> 
>> Giovanni, isn't memoryview just a different _interface_ to the buffer 
>> protocol? 
> 
> To the best of my understanding, it is a more complete interface that
> also allows writable buffers. I think it is the best match to allow
> access to the "contents" of a sip.voidptr and its const version.

Agreed - it's implemented in the hg repo.

>> Anyway, being able to use QSharedMemory with 2.7 and higher 
>> only looks like an arbitrary restriction to me. At least many of my 
>> production systems would be locked out, then. For proper support, Phil 
>> might have to do both variants of the buffer protocol anyway..
> 
> I understand your concerns. Support for memoryview (as a silent
> conversion to/from void* at SIP level) would still be required for
> interoperability with libraries which don't know sip.voidptr but can
> use/produce a memoryview.
> 
> I was just wondering if there was a better solution than coming up with
> a custom API for sip.voidptr for pre-2.7, but I can't see it.

It's not really a custom API - just standard slice and index assignment.

Phil