[Python-ideas] Fwd: Null coalescing operator

[MRAB]

On 2016-09-11 02:02, David Mertz wrote:
> On Sat, Sep 10, 2016 at 5:23 PM, Guido van Rossum <guido at python.org
> <mailto:guido at python.org>> wrote:
>
>     No. PEP 505 actually solves the problem without ever catching
>     AttributeError. Please read it.
>
>
> I read it again (I did a year ago, but reviewed it now).  I hadn't been
> thinking that the *mechanism* of a new None-coalescing operator would
> actually be catching an exception.  It could (and should) work
> differently if it becomes syntax.
>
> What I was getting at with "essentially" was that it would *do the same
> thing* that an AttributeError does.  That is, if `x.foo` can't be
> evaluated (i.e. x doesn't have an attribute 'foo'), then access is
> informally "an error."  The hypothetical "x?.foo" catches that "error"
> and substitutes a different value.  The particular implementation
> under-the-hood is less important for most programmers who might use the
> construct (and I think documentation would actually give an informal
> equivalent as something similar to what I put in the NoneCoalesce class).
>
x?.foo would lookup attribute 'foo' _unless_ x was None, in which case 
it would return None. It's simply:

     None if x is None else x.foo

This means that None?.__str__() would return None, not 'None'. (None has 
an attribute called '__str__', and None.__str__() returns 'None', but it 
would not be looked up because None is, well, None.)