[Python-ideas] Null coalescing operator

[Sven R. Kunze]

On 02.11.2016 17:17, Nick Coghlan wrote:
> The gist is that rather than writing the bare:
>
>     target = expr1 ?? expr2 ?? expr3
>
> You'd instead write None-coalescing as:
>
>     target = exists(expr1) ?? exists(expr2) ?? expr3
>
> and None-propagating as:
>
>     target = missing(expr1) ?? missing(expr2) ?? expr3
>
> with ?? being a protocol-driven short-circuiting binary operator
> controlled by the left operand rather than defining any particular
> semantics of its own.

I am sorry that I had to read this twice to get what you mean.

>
> The "obj?." and "obj?[]" would then be shorthand for particular uses
> of "missing(obj) ?? ..." that avoid duplicate evaluation of the left
> operand (as well as bypassing the overhead of actually creating a
> "missing" instance).


That could work if we accept that ?. and ?[] is only about the left 
hand-side. However, as the ? visually applies also to the 
attribute/index access on the right, which means it could be a more 
readable way of doing

getattr(x, 'attr', None)
or
x[0] if len(x) > 0 else None

Imagine you need to work with incomplete data and do some chaining with 
the new syntax. As soon as one single attribute isn't there, we hit an 
exception. The same for index.

If the ?. and ?[] are introduced, I think applying the "non-existence" 
property also the right hand make sense here as well for a wide range of 
applications.


Best,
Sven