On 7 August 2016 at 15:08, Michael Selik <michael.selik@gmail.com> wrote:


On Sat, Aug 6, 2016 at 8:45 PM INADA Naoki <songofacandy@gmail.com> wrote:
1. bytes(bytearray[:n])
2. bytes(memoryview(bytearray)[:n])

(1) is simplest, but it produces temporary bytearray having n bytes.

Does that actually make the difference between unacceptably inefficient performance and acceptably efficient for an application you're working on?
 
While (2) is more efficient than (1), it uses still temporary memoryview
object, and it looks bit tricky.

Using the memoryview is nicely explicit whereas ``bytes.frombuffer`` could be creating a temporary bytearray as part of its construction.

It could, but it wouldn't (since that would be pointlessly inefficient).

The main question to be answered here would be whether adding a dedicated spelling for "bytes(memoryview(bytearray)[:n])" actually smooths out the learning curve for memoryview in general, where folks would learn:

1. "bytes(mybytearray[:n])" copies the data twice for no good reason
2. "bytes.frombuffer(mybytearray, n)" avoids the double copy
3. "bytes(memoryview(mybytearray)[:n])" generalises to arbitrary slices

With memoryview being a builtin, I'm not sure that argument can be made successfully - the transformation in going from step 1 direct to step 3 is just "wrap the original object with memoryview before slicing to avoid the double copy", and that's no more complicated than using a different constructor method.

Cheers,
Nick.

--
Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia