[Python-ideas] Fwd: Null coalescing operator

[David Mertz]

On Sep 10, 2016 4:45 PM, "Guido van Rossum" <guido at python.org> wrote:
>
> There seems to be a major misunderstanding here. A None-coalescing
> operator is not for catching AttributeError, it's a shortcut similar
> to "a or b" except that it checks for "a is None" rather than bool(a).

That's exactly what the wrapper does. Except it converts all the regular
operators into their None-coalescing versions by putting the extra checks
into the wrapped object itself.

Now admittedly, this DOES mean that the behavior of operations is somewhat
different depending on whether they are wrapped objects or not. False-like
is a different thing than None-like.

This really MUST BE essentially a way a catching AttributeErrors though.
With the proposed syntax 'x?.foo?.bar' will resolve even if x has no 'foo'.
So 'x?.'foo' has to be something special. I guess that special thing could
be a 3.7-style None that responds to new operators, but that's essentially
still *wrapping* a 3.6-style None.

In my mind, as I say, the question marks look ugly, especially when
repeated in chained operations (attribute, call, item get). But even if
they didn't feel bad visually, I don't believe the use case is common
enough to warrant dedicated syntax. Even if my keyboard had some character
I thought was beautiful and intuitive for that meaning, it's still an extra
cognitive burden to distinguish the plain from None-coalescing versions of
every operation, especially for learners.

Another problem is that the question mark still doesn't actually get the
special 'a or b' behavior. For that you still need 'a if a is not None else
b'. Or I guess, in concept, 'a ?or b'. For what it's worth, the wrapper
gives you the special 'a or b' semantics by casting non-Nones as truthy...
But again, 'a' has to have been wrapped first.

>
> On Sat, Sep 10, 2016 at 4:38 PM, David Mertz <mertz at gnosis.cx> wrote:
> > Sorry, I sent this accidentally as private reply, then tried to fix it
on
> > phone.  The latter produced horrible formatting.  Please just read this
> > version.
> >
> > On Sat, Sep 10, 2016 at 4:10 PM, Guido van Rossum <guido at python.org>
wrote:
> >>
> >> So you're offering `NoneCoalesce(x).bar` as less-ugly alternative to
> >> `x?.bar`... Color me unconvinced.
> >
> >
> > No, I'm offering a more realistic use pattern:
> >
> > for x in get_stuff():
> >
> >     x = NoneCoalesce(x)
> >
> >     # ... bunch of stuff with x ...
> >     # ... more stuff with nested keys or attributes ...
> >
> >     x2 = x.foo
> >
> >     x3 = x.bar.baz[x2]
> >
> >     x4 = x(x.val)
> >
> >     result = x3(x4)
> >
> >
> >
> > As a less ugly alternative in the fairly uncommon case that you want
None
> > coalescing as the behavior of getting attributes, keys, call values,
etc.
> > that may or may not be available (AND where you don't want to wrap all
of
> > those access patterns in one try/except block).
> >
> > In contrast, the ugly version of even this pretty simple toy code with
the
> > hypothetical syntax would be:
> >
> > for x in get_stuff():
> >
> >     # ... bunch of stuff with x ...
> >
> >     # ... more stuff with nested keys or attributes ...
> >
> >     x2 = x?.foo
> >
> >     x3 = x?.bar?.baz?[x2]
> >
> >     x4 = x?(x?.val)
> >
> >     result = x3?(x4)
> >
> >
> > This second case looks absolutely awful to me.  And real world uses, if
> > implemented, would quickly get much worse than that.
> >
> > Yours, David...
> >
> > --
> > Keeping medicines from the bloodstreams of the sick; food
> > from the bellies of the hungry; books from the hands of the
> > uneducated; technology from the underdeveloped; and putting
> > advocates of freedom in prisons.  Intellectual property is
> > to the 21st century what the slave trade was to the 16th.
> >
> >
> >
> > --
> > Keeping medicines from the bloodstreams of the sick; food
> > from the bellies of the hungry; books from the hands of the
> > uneducated; technology from the underdeveloped; and putting
> > advocates of freedom in prisons.  Intellectual property is
> > to the 21st century what the slave trade was to the 16th.
> >
> > _______________________________________________
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>
>
>
> --
> --Guido van Rossum (python.org/~guido)
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