On Dec 21, 5:44 pm, Steven D'Aprano <steve +comp.lang.pyt...@pearwood.info> wrote:
> Yes, you should create your lists before trying to append to them. > > But you aren't forced to use a for-loop. You can use a list comprehension: > > x = [some_function(a) for a in range(n)] > > Notice that here you don't need x to pre-exist, because the list comp > creates a brand new list, which then gets assigned directly to x. > > > Now to my actual question. I need to do the above for multiple arrays > > (all the same, arbitrary size). So I do this: > > x=y=z=[] > > This creates one empty list object, and gives it three names, x, y and z. > Every time you append to the list, all three names see the same change, > because they refer to a single list. > > [...] > > > Except it seems that I didn't create three different arrays, I created > > one array that goes by three different names (i.e. x[], y[] and z[] all > > reference the same pile of numbers, no idea which pile). > > Exactly. > > > This surprises me, can someone tell me why it shouldn't? > > Because that's the way Python works. Python is an object-oriented, name > binding language. This is how OO name binding works: you have a single > object, with three names bound to it. The above line is short-cut for: > > a = [] > b = a > c = a > > Python does not make a copy of the list unless you specifically instruct > it to. > > > I figure if I > > want to create and initialize three scalars the just do "a=b=c=7", > > That creates a single integer object with value 7, and binds three names > to it, *exactly* the same as the above. > > If you could modify int objects in place, like you can modify lists in > place, you would see precisely the same effect. But ints are immutable: > all operations on ints create new ints. Lists are mutable, and can be > changed in place. > > > for > > example, so why not extend it to arrays. Also, is there a more pythonic > > way to do "x=[], y=[], z=[]"? > > Well that literally won't work, you can't separate them by commas. > Newlines or semicolons will work. > > Or: x, y, z = [], [], [] > > Either is pretty Pythonic. > > -- > Steven Thanks to you and Dennis for the quick lesson and tips. Very helpful and illuminating. -- http://mail.python.org/mailman/listinfo/python-list
