[Python-Dev] Fw: Behavior of buffer()

[Scott Gilbert]

--- Todd Miller <jmiller@stsci.edu> wrote:
> >
> >I don't understand what you say, but I believe you.
> >
> I meant we call  PyBuffer_FromReadWriteObject and the resulting buffer 
> lives longer than the extension function call that created it.   I have 
> heard that it is possible for the original object to "move" leaving the 
> buffer object pointer to it dangling.

Yes.  The PyBufferObject grabs the pointer from the PyBufferProcs
supporting object when the PyBufferObject is created.  If the PyBufferProcs
supporting object reallocates the memory (possibly from a resize) the
PyBufferObject can be left with a bad pointer.  This is easily possible if
you try to use the array module arrays as a buffer.

I've submitted a patch to fix this particular problem (among others), but
there are still enough things that the buffer object can't do that
something new is needed.

> 
> >
> >
> >>>Maybe instead of the buffer() function/type, there should be a way to
> >>>allocate raw memory?
> >>>
> >
> >>Yes.    It would also be nice to be able to:
> >>
> >>1.  Know (at the python level) that a type supports the buffer C-API.
> >>
> >
> >Good idea.  (I guess right now you can see if calling buffer() with an
> >instance as argument works. :-)
> >
> >>2.  Copy bytes from one buffer to another (writeable buffer).  
> >>

And the copy operations shouldn't create any large temporaries:

  buf1 = memory(50000)
  buf2 = memory(50000)
  # no 10K temporary should be created in the next line
  buf1[10000:20000] = buf2[30000:40000] 

The current buffer object could be used like this, but it would create a
temporary string.  

So getting an efficient copy operation seems to require that slices just
create new "views" to the same memory.

> >
> >Maybe you would like to work on a requirements gathering for a memory
> >object
> >
> Sure.  I'd be willing to poll comp.lang.python (python-list?) and 
> collate the results of any discussion that ensues.  Is that what you had 
> in mind?
> 


In the PEP that I'm drafting, I've been calling the new object "bytes"
(since it is just a simple array of bytes).  Now that you guys are
referring to it as the "memory object", should I change the name?  Doesn't
really matter, but it might avoid confusion to know we're all talking about
the same thing.




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