On Wed, Jul 25, 2018 at 10:12 PM David Mertz <mertz@gnosis.cx> wrote:
On Wed, Jul 25, 2018 at 9:47 PM Nicholas Chammas < nicholas.chammas@gmail.com> wrote:
That is disingenuous, I think. Can this raise an AttributeError?
spam?.eggs?.bacon Of course it can! And this is exactly the pattern used in many
examples in
the PEP and the discussion. So the PEP would create a situation where code will raise AttributeError in a slightly—and subtly—different set of circumstances than plain attribute access will.
food = spam?.eggs?.bacon Can be rewritten as: food = spam if spam is not None and spam.eggs is not None: food = spam.eggs.bacon They both behave identically, no? Maybe I missed the point David was trying to make.
No, you illustrate it perfectly! I had to stare at your translation for a while to decide if it was really identical to the proposed `spam?.eggs?.bacon`. The fact I have to think so hard makes the syntax feel non-obvious.
Plus, there's the fact that your best effort at translating the proposed syntax is WRONG. Even a strong proponent cannot explain the behavior on a first try. And indeed, it behaves subtly different from plain attribute access in where it raises AttributeError.
spam = SimpleNamespace() spam.eggs = None spam.eggs.bacon AttributeError: 'NoneType' object has no attribute 'bacon'
# spam?.eggs?.bacon # Should be: None
"Translation" does something different food = spam if spam is not None and spam.eggs is not None: ... food = spam.eggs.bacon food namespace(eggs=None)
Indeed. Thanks for the counter-example. I think the correct translation is as follows: food = spam?.eggs?.bacon Becomes: food = None if spam is not None and spam.eggs is not None: food = spam.eggs.bacon Did I get it right now? :) What misled me was this example from the PEP <https://www.python.org/dev/peps/pep-0505/#the-maybe-dot-and-maybe-subscript-...> showing how atoms are evaluated. The breakdown begins with `_v = a`, so I copied that pattern incorrectly when trying to explain how an example assignment would work, instead of translating directly from what I understood the PEP 505 variant should do. So, shame on me. I think this particular mistake reflects more on me than on PEP 505, but I see how this kind of mistake reflects badly on the folks advocating for the PEP (or at least, playing devil's advocate).