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

[Guido van Rossum]

On Mon, Sep 28, 2015 at 1:41 PM, Donald Stufft <donald at stufft.io> wrote:

> On September 28, 2015 at 4:25:12 PM, Guido van Rossum (guido at python.org)
> wrote:
> > On Mon, Sep 28, 2015 at 1:15 PM, Donald Stufft wrote:
> >
> > > The ? Modifying additional attribute accesses beyond just the immediate
> > > one bothers me too and feels more ruby than python to me.
> > >
> >
> > Really? Have you thought about it?
>
> Not extensively, mostly this is a gut feeling.
>
> >
> > Suppose I have an object post which may be None or something with a tag
> > attribute which should be a string. And suppose I want to get the
> > lowercased tag, if the object exists, else None.
> >
> > This seems a perfect use case for writing post?.tag.lower() -- this
> > signifies that post may be None but if it exists, post.tag is not
> expected
> > to be None. So basically I want the equivalent of (post.tag.lower() if
> post
> > is not None else None).
> >
> > But if post?.tag.lower() were interpreted strictly as
> (post?.tag).lower(),
> > then I would have to write post?.tag?.lower?(), which is an abomination.
> > OTOH if post?.tag.lower() automatically meant post?.tag?.lower?() then I
> > would silently get no error when post exists but post.tag is None (which
> in
> > this example is an error).
> >
>
> Does ? propagate past a non None value? If it were post?.tag.name.lower()
> and post was not None, but tag was None would that be an error or would the
> ? propagate to the tag as well?
>

I was trying to clarify that by saying that foo?.bar.baz means (foo.bar.baz
if foo is not None else None). IOW if tag was None that would be an error.

The rule then is quite simple: each ? does exactly one None check and
divides the expression into exactly two branches -- one for the case where
the thing preceding ? is None and one for the case where it isn't.

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