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[Python-Dev] Why is pickle.DEFAULT_PROTOCOL still 3?

Brett Cannon brett at python.org
Tue Apr 3 12:42:13 EDT 2018 
On Tue, 3 Apr 2018 at 01:19 Serhiy Storchaka <storchaka at gmail.com> wrote:

> 03.04.18 01:57, Lukasz Langa пише:
> >> On Apr 2, 2018, at 2:13 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> >> On Mon, 2 Apr 2018 13:48:46 -0700
> >> Lukasz Langa <lukasz at langa.pl> wrote:
> >>> Pickle protocol version 4.0 was originally defined back in PEP 3154
> and shipped as part of Python 3.4 back in 2011. Yet it's still not the
> default.
> >> Because we want pickles produced with the default to be readable by
> >> earlier Python 3 versions.
> >> (the same reason protocol 0 stayed the default throughout the Python 2
> >> lifetime)
> >
> > Alright, so that means we can easily do this for Python 3.8, right? I
> mean, following Christian's logic, Python 3.3 is already dead, with its
> final release done in February 2016 and support dropped in September 2017
> per PEP 398.
> >
> > I think we need to get past thinking about "Python 2" vs. "Python 3".
> This frame of mind creates space for another mythical release of Python
> that will break all the compatibilities, something we promised not to do. A
> moving backward compatibility window that includes the last release still
> under security fixes seems like a good new framework for this.
> >
> > What do you think?
>
> The only possible drawback of protocol 4 is that very short pickles can
> be longer than with protocol 3 due to additional 9 bytes for the FRAME
> header and less compact pickling of globals.
>
> This may be partially compensated by implementing additional
> optimizations and/or passing short pickles through pickletools.optimize().
>

I think if you're that worried about specifics to that detail then you
should be specifying the protocol level manually anyway. I view this like
something in the peepholer: a freebie perf bump for those that aren't too
worried about such things. :)
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