[Python-ideas] Null coalescing operators

[Steven D'Aprano]

On Sun, Sep 20, 2015 at 12:05:52PM +0100, Paul Moore wrote:
> On 20 September 2015 at 08:31, Steven D'Aprano <steve at pearwood.info> wrote:
> > I'm not convinced that we should generalise this beyond the three
> > original examples of attribute access, item lookup and function call. I
> > think that applying ? to arbitrary operators is a case of "YAGNI". Or
> > perhaps, "You Shouldn't Need It".
> 
> Agreed.
> 
> Does this need to be an operator? How about the following:

Sadly, I think it does.

Guido has (I think) ruled out the Null object design pattern, which 
makes me glad because I think it is horrid. But your Maybe class below 
is a watered down, weak version that (in my opinion) isn't worth 
bothering with. See below.


class Maybe:
    def __getattr__(self, attr): return None
    def __getitem__(self, idx): return None
    def __call__(self, *args, **kw): return None

def maybe(obj):
    return Maybe() if obj is None else obj


And in action:

py> maybe("spam").upper()  # Works fine.
'SPAM'
py> maybe(None).upper()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable

It also fails for chained lookups:

maybe(obj).spam['id'].ham

will fail for the same reason. You could write this:

maybe(maybe(obj).upper)()
maybe(maybe(maybe(obj).spam)['id']).ham

but that's simply awful. Avoiding that problem is why the Null object 
returns itself, but we've rightly ruled that out.

This is why I think that if this is worth doing, it has to be some sort 
of short-circuiting operator or pseudo-operator:

expression ? .spam.eggs.cheese

can short-circuit the entire chain .spam.eggs.cheese, not just the first 
component. Otherwise, I don't think it's worth doing.



-- 
Steve