On Fri, Jan 13, 2012 at 11:10 PM, Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> wrote: >>>> z = [x, y] # z is a list containing the same sublist twice >>>> z[0].append(23) >>>> print z > [[42, 23], [42, 23]] > > When you work with floats, ints or strings, you don't notice this because > those types are immutable: you can't modify those objects in place. So > for example: > >>>> a = 42 # binds the name 'a' to the object 42 >>>> b = a # a and b point to the same object >>>> a += 1 # creates a new object, and binds it to a >>>> print b # leaving b still pointing to the old object > 42
I was about to say that it's a difference between ".append()" which is a method on the object, and "+=" which is normally a rebinding, but unfortunately: >>> a=[] >>> b=a >>> a+=[1] >>> a [1] >>> b [1] >>> b+=[2] >>> a [1, 2] >>> a [1, 2] >>> a=a+[3] >>> a [1, 2, 3] >>> b [1, 2] (tested in Python 3.2 on Windows) It seems there's a distinct difference between a+=b (in-place addition/concatenation) and a=a+b (always rebinding), which is sorely confusing to C programmers. But then, there's a lot about Python that's sorely confusing to C programmers. ChrisA -- http://mail.python.org/mailman/listinfo/python-list
