On Sun, Sep 11, 2016 at 12:44 PM, Daniel Moisset <dmois...@machinalis.com> wrote: > Both this discussion, PEP 505, and the one a year ago, tend to mix up 2 > related but separate proposals:
I don't think there's that much of a mix-up. PEP 505 clearly describes each proposal separately and even gives a choice to accept or reject each one separately. > (A) Add a None-coalescing operator (like C# a ?? b, what you would write in > Python as "a or b" if it didn't have the falsy gotcha) https://www.python.org/dev/peps/pep-0505/#none-coalescing-operator > (B) Add some None-aware navigation operators ( The "?.", "?()", "?[]", or > what you would write in python as "a and a.attribute" if it didn't have the > falsy gotcha) https://www.python.org/dev/peps/pep-0505/#none-aware-attribute-access-operator > Both are things that can be already done in python, so the purpose here is > to add some convenience (aka "syntax sugar"). IMO, this kind of syntax sugar > proposals should be weighed with the frequency of the coding pattern where > the sugar can be applied. And from the stats presented in PEP-505 (B) is one > order of magnitude less usual than (A); that matches most of the examples I > see in the threads and FWIW my personal experience. I can't argue here yet. Honestly I looked at some examples; for those where it wasn't instantly clear that a None-coalescing operator would *not* help, I found it hard to figure out how to rewrite it. E.g. this one -- quick, is there a better way? return "Illegal Argument" + (self.message is not None and (": " + self.message) or "") I think the answer is, if we had both None-coalescing (??) and None-severing (!!) it could be written as follows: return "Illegal Argument" + ((self.message !! (": " + self.message)) ?? "") but it took me way too long to prove that to myself. > So, as a counterproposal I would like to suggest: > > * Add an "a ?? b" operator which is equivalent to "a if a is None else b" > (but evaluating a once) > * Do not add none-aware navigation; in the less usual scenario where you > need to do it AND ALSO the "and" operator is not usable (it frequently is, > given that by default classes are truish), well, you can use a ternary > operator Honestly the one thing that makes `?.` attractive is that it's easier than the None-coalescing and -severing operators to grasp at an intuitive level. If "foo.bar" raises "AttributeError: 'NoneType' object has no attribute 'foo'" then try again with "foo?.bar". It's surprising how often that will work! > * I don't care if it's an alternate syntax (I'm surprised nobody suggested > "||" which is also used in other languages for similar purpose) Interestingly, after analyzing the above example I desperately want to write it as return "Illegal Argument" + (self.message && (": " + self.message) || "") Note that I already know the relative priorities of && and ||, so I can drop a set of parentheses. > Would this satisfy most of the people requesting this? (and, would it > satisfy the people making the decision?) Personally, after the above example, I'm less excited about ??/!! or ||/&&, and more excited about `?.` -- so it doesn't satisfy me. -- --Guido van Rossum (python.org/~guido) _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
