candide wrote: > Suppose a and b are lists. > > What is more efficient in order to extend the list a by appending all > the items in the list b ? > > > I imagine a.extend(b)to be more efficient for only appendinding the > items from b while a+=b creates a copy of a before appending, right ?
No. Both append items to the original list a: >>> a = original_a = [1,2,3] >>> a.extend([4,5,6]) >>> a is original_a # you knew that True >>> a += [7,8,9] # could rebind a >>> a is original_a # but effectively doesn't for lists True (Strictly speaking a += [...] rebinds a to the same value, like a = a) It is mostly a matter of personal preference which form you use. I prefer the extend() method because I think it's clearer; you don't run the risk of mistaking it for an arithmetic operation. Peter PS: an example where += does rebind: >>> a = original_a = (1,2,3) >>> a += (4,5,6) >>> a is original_a False -- http://mail.python.org/mailman/listinfo/python-list
