[Python-Dev] Tweak to PEP 523 for storing a tuple in co_extra

[Brett Cannon]

On Sat, 3 Sep 2016 at 16:55 Yury Selivanov <yselivanov.ml at gmail.com> wrote:

>
>
> On 2016-09-03 4:13 PM, Chris Angelico wrote:
> > On Sun, Sep 4, 2016 at 8:03 AM, Yury Selivanov <yselivanov.ml at gmail.com>
> wrote:
> >> On 2016-09-03 12:27 PM, Brett Cannon wrote:
> >>> Below is the `co_extra` section of PEP 523 with the update saying that
> >>> users are expected to put a tuple in the field for easier simultaneous
> use
> >>> of the field.
> >>>
> >>> Since the `co_extra` discussions do not affect CPython itself I'm
> planning
> >>> on landing the changes stemming from the PEP probably on Monday.
> >>
> >> Tuples are immutable.  If you have multiple co_extra users then they
> will
> >> have to either mutate tuple (which isn't always possible, for instance,
> you
> >> can't increase size), or to replace it with another tuple.
> > Replace it, but only as they register themselves with a particular
> > function. Imagine a profiler doing something vaguely like this:
>
> "Replacing" makes it error prone to cache the pointer even for small
> periods of time. Defining co_extra using Python C API forces us to
> acquire the GIL etc (aside from other performance penalties). Although
> we probably would recommend to use the GIL anyways, I'm not sure tuple
> really simplifies anything here.
>
> >
> > class FunctionStats:
> >      def __init__(self):
> >          self.info = [whatever, whatever, blah blah]
> >
> > def profile(func):
> >      """Decorator to mark a function for profiling"""
> >      func.__code__.co_extra += (FunctionStats(),)
> >      return func
> >
> > Tuple immutability impacts the initialization only. After that, you
> > just iterate over it.
>
> I wasn't aware we wanted to expose co_extra to Python land.
>

We are most definitely not exposing the field to Python code.
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