On Tue, Jul 31, 2018 at 02:14:19PM -0400, David Mertz wrote:
On Tue, Jul 31, 2018, 1:47 PM Steven D'Aprano <steve@pearwood.info> wrote:
Yes it is. Rhodri is correct, although I admit that I hadn't realised this point myself until he pointed it out. (That is why until now I've been writing examples like "spam?.eggs?.cheese?.aardvark" which is redundant.)
Again, one of the most vocal advocates of the PEP gets the semantics wrong!
`spam?.eggs?.cheese?.aardvark` is NOT redundant for ` spam?.eggs.cheese.aardvark`.
It is if *only* spam can be None, which is the scenario being discussed. Please don't ignore the context of statements. I didn't say that spam?.eggs.cheese.aardvark would guard against eggs or cheese being None. That interpretation of my comment is uncharitable and not supported by my re-write of the pseudo-code from the PEP. It also goes completely against my comment "Figuratively speaking..." that the ?. operator groups to the right: spam?(.eggs.cheese.aardvark) # grouping, not function call. Perhaps you didn't read my post all the way to the end before firing off a triumphant post accusing me of getting it wrong. In context, the question is whether: spam?.eggs.cheese.aardvark might attempt to evaluate None.cheese.aardvark. It won't. In context, the issue was not whether these two expressions are identical in every imaginable way: spam?.eggs.cheese.aardvark spam?.eggs?.cheese?.aardvark In pseudo-code, they would be something close to: if spam is not None: return spam.eggs.cheese.aardvark versus if spam is not None: if spam.eggs is not None: if spam.eggs.cheese is not None: return spam.eggs.cheese.aardvark except without needing to evaluate each subexpression multiple times. In context, we are discussing the behaviour when spam is None. When spam is None, there's no need for the additional maybe-dots, because the first short-circuits the rest. If additional attributes also could be None, then they too could be guarded with maybe-dot operators, but if they are never None, it is unnecessary and redundant to guard spam alone all the way through the expression with maybe-dots.
The two expressions simply do different things, but in a way guaranteed to assure that also everyone gets the actual behaviors wrong.
Putting aside your prejudicial language about guaranteeing errors, I think that the PEP could be improved. I managed to read it multiple times without taking in the fact that ?. short-circuits to the right. I didn't explicitly say so (perhaps I should have) but I too initially made the same error as Abe, assuming that spam?.eggs.cheese would fail if spam was None (evaluating None.cheese) but according to the PEP that's not the case. With a working implementation, it would be the work of moments for people to experiment in the REPL and clarify any lingering doubts about the operation. Without it, there will always be some misunderstandings from those who (like me) failed to read the documentation carefully enough and made upjustified assumptions. That's no different from any other non-trivial feature. -- Steve