[Python-ideas] PEP 505 -- None-aware operators

[Franklin? Lee]

My mistake. You're talking about the ?? operator, and I'm thinking about
the null-aware operators.

You say short-circuiting would be nice to have, but short-circuiting is
what people want it for. As for using `if-else`, that's listed as an
alternative here:
https://www.python.org/dev/peps/pep-0505/#ternary-operator

The coalesce operator has the semantic advantage ("expressiveness"?): you
are saying what you want to do, rather than how you do it. Making a
function is one way to get semantic advantage, but you can't do that if you
want short-circuiting.

The question on the table is whether the semantic advantage is worth the
cost of a new operator. That's a value question, so it's not gonna be easy
to answer it with objective observations.

(Not that I'm suggesting anything, but some languages have custom
short-circuiting, via macros or lazy arg evalation. That's be using a
missile to hammer in a nail.)

On Oct 14, 2016 9:46 AM, "Franklin? Lee" <leewangzhong+python at gmail.com>
wrote:
>
> On Oct 14, 2016 9:14 AM, "Gustavo Carneiro" <gjcarneiro at gmail.com> wrote:
> >
> > Sorry if I missed the boat, but only just now saw this PEP.
> >
> > Glancing through the PEP, I don't see mentioned anywhere the SQL
alternative of having a coalesce() function:
https://www.postgresql.org/docs/9.6/static/functions-conditional.html#FUNCTIONS-COALESCE-NVL-IFNULL
> >
> > In Python, something like this:
> >
> > def coalesce(*args):
> >     for arg in args:
> >         if arg is not None:
> >              return arg
> >     return None
> >
> > Just drop it into builtins, and voila.   No need for lengthy
discussions about which operator to use because IMHO it needs no operator.
> >
> > Sure, it's not as sexy as a fancy new operator, nor as headline
grabbing, but it is pretty useful.
>
> That function is for a different purpose. It selects the first non-null
value from a flat collection. The coalesce operators are for traveling down
a reference tree, and shortcutting out without an exception if the path
ends.
>
> For example:
>     return x?.a?.b?.c
> instead of:
>     if x is None: return None
>     if x.a is None: return None
>     if x.a.b is None: return None
>     return x.a.b.c
>
> You can use try-catch, but you might catch an irrelevant exception.
>     try:
>         return x.a.b.c
>     except AttributeError:
>         return None
> If `x` is an int, `x.a` will throw an AttributeError even though `x` is
not None.
>
> A function for the above case is:
>     def coalesce(obj, *names):
>         for name in names:
>             if obj is None:
>                 return None
>             obj = getattr(obj, name)
>         return obj
>
>     return coalesce(x, 'a', 'b', 'c')
>
> See this section for some examples:
> https://www.python.org/dev/peps/pep-0505/#behavior-in-other-languages
>
> (The PEP might need more simple examples. The Motivating Examples are
full chunks of code from real libraries, so they're full of distractions.)
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