[Python-ideas] PEP 505 (None coalescing operators) thoughts

[MRAB]

On 2015-09-28 21:06, Guido van Rossum wrote:
> On Mon, Sep 28, 2015 at 1:00 PM, Carl Meyer <carl at oddbird.net
> <mailto:carl at oddbird.net>> wrote:
>
>     On 09/28/2015 01:53 PM, Guido van Rossum wrote:
>     > On Mon, Sep 28, 2015 at 12:43 PM, Carl Meyer <carl at oddbird.net <mailto:carl at oddbird.net>
>     >     Or put differently, that whereas these two are trivially equivalent (the
>     >     definition of left-to-right binding within a precedence class):
>     >
>     >         foo.bar.baz
>     >         (foo.bar).baz
>     >
>     >     these two are not equivalent:
>     >
>     >        foo?.bar.baz
>     >        (foo?.bar).baz
>     >
>     >
>     > Right.
>     >
>     >
>     >     I'm having trouble coming up with a parallel example where the existing
>     >     short-circuit operators break "extractibility" of a sub-expression like
>     >     that.
>     >
>     >
>     > Why is that an interesting property?
>
>     Because breaking up an overly-complex expression into smaller
>     expressions by means of extracting sub-expressions into temporary
>     variables is a common programming task (in my experience anyway --
>     especially when trying to decipher some long-gone programmer's
>     overly-complex code), and it's usually one that can be handled pretty
>     mechanically according to precedence rules, without having to consider
>     that some operators might have action-at-a-distance beyond their
>     precedence.
>
>
> Well, if just the foo?.bar.baz part is already too complex you probably
> need to reconsider your career. :-)
>
> Seriously, when breaking things into smaller parts you *have* to
> understand the shortcut properties. You can't break "foo() or bar()" into
>
>    a = foo()
>    b = bar()
>    return a or b
>
> either.
>
Exactly.

Can you break:

     result = do_this() if test() else do_that()

into parts without changing its meaning/behaviour?

     condition = test()
     true_result = do_this()
     false_result = do_that()
     result = true_result if condition else false_result

The ? 'operators' are syntactic sugar.

>     >     I guess this is because the proposed short-circuiting still "breaks out
>     >     of the precedence order" in a way that the existing short-circuiting
>     >     operators don't. Both member access and indexing are within the same
>     >     left-to-right binding precedence class, but the new operators would have
>     >     a short-circuit effect that swallows operations beyond where normal
>     >     left-to-right binding would suggest their effect should reach.
>     >
>     >     Are there existing examples of behavior like this in Python that I'm
>     >     missing?
>     >
>     >
>     > I don't know, but I think you shouldn't worry about this.
>
>     I think it's kind of odd, but if nobody else is worried about it, I
>     won't worry about it either :-)
>
>
> Good idea.
>