On Sat, May 1, 2010 at 10:05 PM, Alf P. Steinbach <al...@start.no> wrote: > On 02.05.2010 06:06, * Aahz: >> In article<4bdcd631$0$27782$c3e8...@news.astraweb.com>, >> Steven D'Aprano<st...@remove-this-cybersource.com.au> wrote: >>> >>> On Sat, 01 May 2010 07:13:42 -0500, Tim Chase wrote: >>>> >>>> The += family of operators really do rebind the symbol, not modify the >>>> object. >>> >>> They potentially do both, depending on the object, even for built-ins. >> >> No, they always rebind; sometimes they modify the object > > If they always rebind and sometimes modify object then they "potentially do > both", and so the "No" at the start of the sentence contradicts this later > part. > > >> and sometimes >> they rebind the original target to the same object. > > At the Python level that seems to be an undetectable null-operation. Granted > one could see something going on in a machine code or byte code debugger. > But making that distinction (doing nothing versus self-assignment) at the > Python level seems, to me, to be meaningless.
There are some circumstances where the subtle distinction matters. Consider x.y += z where x is an instance of a class that overrides __setattr__ and __getattribute__ and x.y results in a mutable object. Not doing the assignment can result in a noticeable difference in behavior since __setattr__ won't get called. Yes, this is a slightly obscure case, but it does come up. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
