[Python-ideas] Fwd: Null coalescing operator

[David Mertz]

Ok, I have been thinking of the behavior too broadly. I realize now that
`x?.foo` might still simply raise an AttributeError if x is neither None
nor a thing with a foo attribute.

The class I wrote is definitely too aggressive for the behavior described.
On the other hand, by being narrower in behavior there feels like even less
motivation for new syntax.

On Sep 10, 2016 6:29 PM, "MRAB" <python at mrabarnett.plus.com> wrote:

> 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.)
>
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