[Python-ideas] PEP 505: None-aware operators

[MRAB]

On 2018-07-31 20:53, Jonathan Fine wrote:
> Stephan Houben wrote:
> 
>> Nope, the introduction of the tmp variable changed the semantics. It isn't a
>> "chain" anymore so it breaks shortcutting.
> 
> I'm confused. Assume 'a' is not defined.
> 
> With Python's dot (attribute access) we have
>>>> a.b.c
> NameError: name 'a' is not defined
>>>> a.(b.c)
> SyntaxError: invalid syntax
>>>> (a.b).c
> NameError: name 'a' is not defined
> 
> I'd expect, after PEP 505 is added to Python that we'd get
>>>> a ?. b ?. c
> NameError: name 'a' is not defined
>>>> a ?. (b ?. c)
> SyntaxError: invalid syntax
>>>> (a ? . b) ?. c
> NameError: name 'a' is not defined
> 
> If this is not correct, please some do tell me what we would get.
> 
Correct.

> Now assume 'a' is defined. I'd also expect, for any value for 'a', that
>>>> tmp = (a ?. b)
>>>> val = tmp ?. c
> give val the same value as
>>>> val = (a ?. b) ?. c
> 
> If this is not so, then can the second val be computed from tmp? And if so, how?
> 
Also correct.

On the first point, Steven said that _figuratively speaking_ (his words) 
the series of attribute accesses would be _as though_ it was grouped 
a?.(b.c), so if a is None, then remainder of the attributes accessed 
would be short-circuited. He _wasn't_ suggesting that that was _actual_ 
syntax.

On the second point, you'd said:

     tmp = spam ?. eggs
     val2 = tmp . cheese . aardvark    # For spam?.eggs.cheese.aardvark

i.e. that:

     spam?.eggs.cheese.aardvark

could be rewritten as:

     tmp = spam ?. eggs
     val2 = tmp . cheese . aardvark

Steven pointed out that that was wrong.

In the first, if spam is None, then the remainder is short-circuited, 
and so the result of spam?.eggs.cheese.aardvark is None.

In the second, if spam is None, then tmp would be None, and tmp.cheese 
would fail.