On Thu, 02 Feb 2012 12:25:00 -0500 Terry Reedy <tjre...@udel.edu> wrote:
> On 2/2/2012 9:17 AM, John O'Hagan wrote: > > > It's not so much about the type of x but that of x[1]. Wouldn't it > > be possible to omit the assignment simply if the object referred to > > by x[1] uses "+=" without creating a new object? That way, > > some_tuple[i] += y will succeed if some_tuple[i] is a list but not > > with, say, an int. That seems reasonable to me. > > There was considerable discussion of the exact semantics of augmented > operations when they were introduced. I do not remember if that > particular idea was suggested (and rejected) or not. You could try to > look at the PEP, if there is one, or the dicussion ( probably on > pydev list). > I think we're 12 years late on this one. It's PEP 203 from 2000 and the key phrase was: "The in-place function should always return a new reference, either to the old `x' object if the operation was indeed performed in-place, or to a new object." If this had read: "The in-place function should return a reference to a new object if the operation was not performed in-place." or something like that, we wouldn't be discussing this. The discussion on py-dev at the time was quite limited but there was some lively debate on this list the following year (in the context of widespread controversy over new-fangled features which also included list comprehensions and generators), to which the BDFL's response was: "You shouldn't think "+= is confusing because sometimes it modifies an object and sometimes it does". Gee, there are lots of places where something that's *spelled* the same has a different effect depending on the object types involved." That's true, but I don't think there should be a different effect depending on what _name_ we use for an operand: >>> t=([],) >>> l=t[0] >>> l is t[0] True >>> l+=[1] >>> t ([1],) >>> t[0]+=[1] Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment >>> t ([1, 1],) >>> l is t[0] True Same object, same operator, different name, different outcome. Maybe that was obvious from the foregoing discussion, but it shocked me when put that way. John -- http://mail.python.org/mailman/listinfo/python-list
