Bugs item #455694, was opened at 2001-08-27 08:15 Message generated for change (Comment added) made by gbrandl You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=455694&group_id=5470
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: Documentation Group: None Status: Closed Resolution: Fixed Priority: 5 Private: No Submitted By: Nobody/Anonymous (nobody) Assigned to: Fred L. Drake, Jr. (fdrake) Summary: bug in list append, or list multiply? Initial Comment: Python 2.1.1 (Windows NT 4) >>> x=6*[[]] >>> x [[], [], [], [], [], []] >>> x[3].append(7) >>> x [[7], [7], [7], [7], [7], [7]] But, I was expecting: [[], [], [], [7], [], []] So I then tried this: >>> x=[[],[],[],[],[],[]] # instead of x=6*[[]] >>> x[3].append(7) >>> x [[], [], [], [7], [], []] And it worked. ---- I imagine what is happening is that 6*[[]] creates 6 pointers to the same empty list? But, if so, the python docs (http://www.python.org/doc/current/lib/typesseq.html) imply otherwise: s * n , n * s yields n _copies_ of s concatenated Anyway, I'm confused. ---------------------------------------------------------------------- >Comment By: Georg Brandl (gbrandl) Date: 2007-02-09 18:50 Message: Logged In: YES user_id=849994 Originator: NO This behavior is not a bug, it is how objects in Python and the '*' operator for lists work. ---------------------------------------------------------------------- Comment By: M.Hampton (hamptonio) Date: 2007-02-09 11:43 Message: Logged In: YES user_id=1373289 Originator: NO This really messed me recently, and I consider it a definite bug. It is still present in python 2.5 on multiple platforms. ---------------------------------------------------------------------- Comment By: Fred L. Drake, Jr. (fdrake) Date: 2001-08-28 14:56 Message: Logged In: YES user_id=3066 Fixed in Doc/lib/libstdtypes.tex revision 1.68. ---------------------------------------------------------------------- Comment By: Tim Peters (tim_one) Date: 2001-08-27 21:25 Message: Logged In: YES user_id=31435 Changed to Doc and reassigned to Fred. The docs may be clearer if they said "n shallow copies" instead of "n copies". >>> s = [[]] >>> from copy import copy >>> x = copy(s) + copy(s) + copy(s) >>> x [[], [], []] >>> x[1].append(7) >>> x [[7], [7], [7]] >>> That is, "s*n is n copies of s concatenated" is correct, but only if you have shallow copies in mind. Anonymous, the effect you *want* can be gotten via x = [] for i in range(6): x.append([]) or, in Python 2.0+ via x = [[] for i in range(6)] Doing [O] * n creates a list with n repetitions of O, i.e. exactly the same object n times, the same as [O, O, O, ...]. In [[], []] you create two distinct empty lists, but in temp = [] [temp, temp] you get a single empty list twice. Same thing for [[]] * n ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=455694&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
