On 2015-09-28 21:06, Guido van Rossum wrote:
On Mon, Sep 28, 2015 at 1:00 PM, Carl Meyer <carl@oddbird.net <mailto:carl@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@oddbird.net <mailto:carl@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.