On Feb 7, 3:13 pm, "Gabriel Genellina" <[EMAIL PROTECTED]> wrote: > En Thu, 07 Feb 2008 16:16:11 -0200, [EMAIL PROTECTED] > <[EMAIL PROTECTED]> escribió: > > > On Feb 7, 11:38 am, [EMAIL PROTECTED] wrote: > > I don't see why you should get either. > > > Especially considering this behaviour: > > >>>> a=[] > >>>> row=[ [] for n in range(0,10) ] > >>>> a.extend(row[:]) > >>>> a > > [[], [], [], [], [], [], [], [], [], []] > >>>> a[0].extend(row[:]) > >>>> a > > [[[...], [], [], [], [], [], [], [], [], []], [], [], [], [], [], [], > > [], [], []] > > Those [...] should give a clue. Usually Python doesn't "shorten" a list > representation: if it takes a thousand lines to output a list, there will > be a thousand lines of output. The [...] means that the list is > *recursive*: it has an element that refers to the list itself, so it can't > be represented in the normal way. > Why is it recursive? row[:] is a new list, not the same object as row. It > is a copy - but a "shallow" copy, because their elements aren't copies > themselves. So row[0] is the same object as row[:][0] > > >>> x = row[:] > >>> x == row > True > >>> x is row > False > >>> x[0] is row[0] > > True > > In the line a[0].extend(row[:]), a[0] is THE SAME LIST as row[:][0], the > first item you are appending. That is, the first thing extend() does is > conceptually a[0].append(a[0]) - and you got a recursive structure.
I thought that it was some kind of recursive hocus pocus. That explains why no matter how many indexes I ask for, I always get the same result. >>> a[0][0][0][0][0][0][0][0][0][0][0][0][0][0] [[...], [], [], [], [], [], [], [], [], []] > > > Bug in IDLE? > > No, just a misunderstanding of what [:] does, I presume. You didn't answer my question, but I just realize that I mis-typed my example, so never mind. > > -- > Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
