[Python-ideas] Null coalescing operators
steve at pearwood.info
Mon Sep 21 06:06:19 CEST 2015
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".
> 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.
def __getattr__(self, attr): return None
def __getitem__(self, idx): return None
def __call__(self, *args, **kw): return None
return Maybe() if obj is None else obj
And in action:
py> maybe("spam").upper() # Works fine.
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable
It also fails for chained lookups:
will fail for the same reason. You could write this:
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.
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