On Thu, 20 Dec 2012 18:59:39 -0500, Terry Reedy wrote: >> On Thu, Dec 20, 2012 at 8:37 PM, Pierre Quentel >> <pierre.quen...@gmail.com> wrote: >>> I'm afraid I am going to disagree. The document is a tree structure, >>> and today Python doesn't have a syntax for easily manipulating trees. > > What Python does have is 11 versions of the augmented assignment > statement: +=, -=, *=, /=, //=, %=, **=, >>=, <<=, &=, ^=, |=. Moreover, > these are *intended* to be implemented in place, by mutation, for > mutable objects, with possibly class-specific meanings.
I don't believe that is the case. The problem is that augmented assignment that mutates can be rather surprising to anyone who expects "a += b" to be a short cut for "a = a + b". py> a = [1, 2, 3]; b = [99]; another = a py> a = a + b py> print(a, another) # What I expect. [1, 2, 3, 99] [1, 2, 3] py> a = [1, 2, 3]; b = [99]; another = a py> a += b py> print(a, another) # Surprise! [1, 2, 3, 99] [1, 2, 3, 99] Whichever behaviour you pick, you're going to surprise somebody. So I wouldn't say that mutate in place is *intended* or preferred in any way, only that it is *allowed* as an optimization if the class designer prefers so. One might even have a class where (say) __iadd__ is defined but __add__ is not. [...] > <= is a comparison expression operator, which is completely different. <= is a comparison operator for ints, floats, strings, lists, ... but not necessarily for *everything*. That's the beauty and horror of operator overloading. Any operator can mean anything. If it were intended to only return a flag, then 1) Python would enforce that rule, and 2) the numpy people would be most upset. I have no opinion on the usefulness or sensibility of using <= as an in- place mutator method in this context, but I will say that if I were designing my own mini-DSL, I would not hesitate to give "comparison operators" some other meaning. Syntax should be judged in the context of the language you are using, not some other language. If you are using a DSL, then normal Python rules don't necessarily apply. <= in particular looks just like a left-pointing arrow and is an obvious candidate for overloading. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
