[Python-Dev] PEP 554 v3 (new interpreters module)

Nathaniel Smith njs at pobox.com
Thu Sep 14 01:27:19 EDT 2017

On Sep 13, 2017 9:01 PM, "Nick Coghlan" <ncoghlan at gmail.com> wrote:

On 14 September 2017 at 11:44, Eric Snow <ericsnowcurrently at gmail.com>
>    send(obj):
>        Send the object to the receiving end of the channel.  Wait until
>        the object is received.  If the channel does not support the
>        object then TypeError is raised.  Currently only bytes are
>        supported.  If the channel has been closed then EOFError is
>        raised.

I still expect any form of object sharing to hinder your
per-interpreter GIL efforts, so restricting the initial implementation
to memoryview-only seems more future-proof to me.

I don't get it. With bytes, you can either share objects or copy them and
the user can't tell the difference, so you can change your mind later if
you want. But memoryviews require some kind of cross-interpreter strong
reference to keep the underlying buffer object alive. So if you want to
minimize object sharing, surely bytes are more future-proof.

> Handling an exception
> ---------------------
> ::
>    interp = interpreters.create()
>    try:
>        interp.run("""if True:
>            raise KeyError
>            """)
>    except KeyError:
>        print("got the error from the subinterpreter")

As with the message passing through channels, I think you'll really
want to minimise any kind of implicit object sharing that may
interfere with future efforts to make the GIL truly an *interpreter*
lock, rather than the global process lock that it is currently.

One possible way to approach that would be to make the low level run()
API a more Go-style API rather than a Python-style one, and have it
return a (result, err) 2-tuple. "err.raise()" would then translate the
foreign interpreter's exception into a local interpreter exception,
but the *traceback* for that exception would be entirely within the
current interpreter.

It would also be reasonable to simply not return any value/exception from
run() at all, or maybe just a bool for whether there was an unhandled
exception. Any high level API is going to be injecting code on both sides
of the interpreter boundary anyway, so it can do whatever exception and
traceback translation it wants to.

> Reseting __main__
> -----------------
> As proposed, every call to ``Interpreter.run()`` will execute in the
> namespace of the interpreter's existing ``__main__`` module.  This means
> that data persists there between ``run()`` calls.  Sometimes this isn't
> desireable and you want to execute in a fresh ``__main__``.  Also,
> you don't necessarily want to leak objects there that you aren't using
> any more.
> Solutions include:
> * a ``create()`` arg to indicate resetting ``__main__`` after each
>   ``run`` call
> * an ``Interpreter.reset_main`` flag to support opting in or out
>   after the fact
> * an ``Interpreter.reset_main()`` method to opt in when desired
> This isn't a critical feature initially.  It can wait until later
> if desirable.

I was going to note that you can already do this:


However, that turns out to clear *too* much, since it also clobbers
all the __dunder__ attributes that the interpreter needs in a code
execution environment.

Either way, if you added this, I think it would make more sense as an
"importlib.util.reset_globals()" operation, rather than have it be
something specific to subinterpreters.

This is another point where the API could reasonably say that if you want
clean namespaces then you should do that yourself (e.g. by setting up your
own globals dict and using it to execute any post-bootstrap code).

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