On Wednesday, August 15, 2012 8:21:22 PM UTC-4, Terry Reedy wrote: > On 8/15/2012 5:58 PM, Rob Day wrote: > > Yeah, my apologies for any confusion I created. Although I suppose my > explanation would be somewhat true for immutable objects since they can't be > modified in-place (any modification at all would cause the creation of a new > immutable object right?), I now understand that it is completely and totally > wrong for mutable objects.
Thanks for the in-depth explanations, Terry and Rob. I feel like I have a much more solid grasp of what's going on behind the scenes after your analysis. > > > Madison May wrote: > > > The list nlist inside of function xx is not the same as the variable > > > u outside of the function: nlist and u refer to two separate list > > > objects. When you modify nlist, you are not modifying u. > > > <http://mail.python.org/mailman/listinfo/python-list> > > > > This is confused and wrong. The parameter *name* 'nlist' of function xx > > is not the same as the *name* 'u' outside the function. The call xx(u) > > binds nlist to the same object that u is bound to. At that point, the > > two name *are* bound to the same list object. The statement > > "nlist+=[999]" dodifying nlist *does* modify u. The subsequent > > assignment statement "nlist=nlist[:-1]" rebinds 'nlist' to a *new* list > > object. That new object gets deleted when the function returns. So the > > rebinding is completely useless. > > > > This sequence, modifying the input argument and then rebinding to a new > > object, is bad code. > > > > > Well - that's not quite true. Before calling the function, u is [1, 2, > > > 3, 4] - but after calling the function, u is [1, 2, 3, 4, 999]. This is > > > a result of using 'nlist += [999]' - the same thing doesn't happen if > > > you use 'nlist = nlist+[999]' instead. > > > > > > I'm not completely aware of what's going on behind the scenes here, but > > > > you got it right. > > > > > I think the problem is that 'nlist' is actually a reference to a list > > > object - it points to the same place as u. > > > > Calling a python function binds parameter names to argument objects or > > (for *args and **kwds parameters) a collection based on argument objects. > > > > > When you assign to it within > > > the function, then it becomes separate from u - which is why nlist = > > > nlist+[999] and nlist = nlist[:-1] don't modify u - but if you modify > > > nlist in place before doing that, such as by using +=, then it's still > > > pointing to u, and so u gets modified as well. > > > > > > -- > > Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
