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

[Guido van Rossum]

On Mon, Sep 28, 2015 at 12:53 PM, Carl Meyer <carl at oddbird.net> wrote:

> On 09/28/2015 01:43 PM, Carl Meyer wrote:
> [snip]
> > I assume that the short-circuiting would follow the precedence
> > order; that is, nothing with looser precedence than member and index
> > access would be short-circuited. So, for example,
> >
> >     foo?.bar['baz'].spam
> >
> > would short-circuit the indexing and the final member access,
> translating to
> >
> >     foo.bar['baz'].spam if foo is not None else None
> >
> > but
> >
> >     foo?.bar or 'baz'
> >
> > would mean
> >
> >     (foo.bar if foo is not None else None) or 'baz'
> >
> > and would never evaluate to None. Similarly for any operator that binds
> > less tightly than member/index access (which is basically all Python
> > operators).
>
> For a possibly less-intuitive example of this principle (arbitrarily
> picking the operator that binds next-most-tightly), what should
>
>     foo?.bar**3
>
> mean?
>

It's nonsense -- it means (foo?.bar)**3 but since foo?.bar can return None
and None**3 is an error you shouldn't do that. But don't try to then come
up with syntax that rejects foo?.bar**something statically, because
something might be an object implements __rpow__.

And I still don't see why this "principle" would be important.

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