[Python-ideas] PEP 505 (None coalescing operators) thoughts

[Guido van Rossum]

On Mon, Sep 28, 2015 at 12:47 PM, Andrew Barnert <abarnert at yahoo.com> wrote:

> On Monday, September 28, 2015 12:05 PM, Guido van Rossum <guido at python.org>
> wrote:
> >On Mon, Sep 28, 2015 at 10:38 AM, Carl Meyer <carl at oddbird.net> wrote:
> >>"Propagating" refers to the proposed behavior where use of ?. or ?[
> >>"propagates" through the following chain of operations. For example:
> >>
> >>    x = foo?.bar.spam.eggs
> >>
> >>Where both `.spam` and `.eggs` would behave like `?.spam` and `?.eggs`
> >>(propagating None rather than raising AttributeError), simply because a
> >>`.?` had occurred earlier in the chain. So the above behaves differently
> >>from:
> >>
> >>    temp = foo?.bar
> >>    x = temp.spam.eggs
> >>
> >>Which raises questions about whether the propagation escapes
> >>parentheses, too:
> >>
> >>    x = (foo?.bar).spam.eggs
> >>
> >
> >Oh, I see. That's evil.
> >
> >The correct behavior here is that "foo?.bar.spam.eggs" should mean the
> same as
> >
> >    (None if foo is None else foo.bar.spam.eggs)
> >
> >(Stop until you understand that is *not* the same as either of the
> alternatives you describe.)
> >
> >I can see the confusion that led to the idea of "propagation" -- it
> probably comes from an attempt to define "foo?.bar" without reference to
> the context (in this case the relevant context is that it's followed by
> ".spam.eggs").
>
>
> It would really help to have a complete spec, or at least a quick
> workthrough of how an expression gets parsed and compiled.
>

Isn't the PEP author still planning to do that? But it hasn't happened yet.
:-(


> I assume it's something like this:
>
> spam?.eggs.cheese becomes this pseudo-AST (I've skipped the loads and
> maybe some other stuff):
>
>     Expr(
>         value=Attribute(
>             value=Attribute(
>                 value=Name(id='spam'), attr='eggs', uptalk=True),
>             attr='cheese', uptalk=False))
>

Hm, I think the problem is that this way of representing the tree
encourages thinking that each attribute (with or without ?) can be treated
on its own.

… which is then compiled as this pseudo-bytecode:
>
>     LOAD_NAME 'spam'
>     DUP_TOP
>     POP_JUMP_IF_NONE :label
>     LOAD_ATTR 'eggs'
>     LOAD_ATTR 'cheese'
>     :label
>
>
> I've invented a new opcode POP_JUMP_IF_NONE, but it should be clear what
> it does. I think it's clear how replacing spam with any other expression
> works, and how subscripting works. So the only question is whether
> understanding how .eggs.cheese becomes a pair of LOAD_ATTRs is sufficient
> to understand how ?.eggs.cheese becomes a JUMP_IF_NONE followed by the same
> pair of LOAD_ATTRs through the same two steps.
>

To most people of course that's indecipherable mumbo-jumbo. :-)


> I suppose the reference documentation wording is also important here, to
> explain that an uptalked attributeref or subscription short-circuits the
> whole primary.
>

Apparently clarifying that is the entire point of this thread. :-)

-- 
--Guido van Rossum (python.org/~guido)
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