[issue8023] bug in s.append(x)
New submission from ughacks ugha...@yahoo.com: Dear, I am using $ python -V Python 2.6.4 on Ubuntu 9.10 I met a serious bug in s.append(x) operation. If I append a list into another list, there is a change of content. In the following code, [2,-2,0,0] is replaced with [-2,-2,0,0] after s.append(x) operaton. --- begin of code -- total=[] for i in range(4): for j in range(i): root=[0,0,0,0] for k in [2,-2]: for l in [2,-2]: root[i]=k root[j]=l total.append(root) print root print total --- end of code - Result: each element is correctly generated [2, 2, 0, 0] [-2, 2, 0, 0] [2, -2, 0, 0] [-2, -2, 0, 0] [2, 0, 2, 0] [-2, 0, 2, 0] [2, 0, -2, 0] [-2, 0, -2, 0] [0, 2, 2, 0] [0, -2, 2, 0] [0, 2, -2, 0] [0, -2, -2, 0] [2, 0, 0, 2] [-2, 0, 0, 2] [2, 0, 0, -2] [-2, 0, 0, -2] [0, 2, 0, 2] [0, -2, 0, 2] [0, 2, 0, -2] [0, -2, 0, -2] [0, 0, 2, 2] [0, 0, -2, 2] [0, 0, 2, -2] [0, 0, -2, -2] But the total list is wrong [[-2, -2, 0, 0], [-2, -2, 0, 0], [-2, -2, 0, 0], [-2, -2, 0, 0], [-2, 0, -2, 0], [-2, 0, -2, 0], [-2, 0, -2, 0], [-2, 0, -2, 0], [0, -2, -2, 0], [0, -2, -2, 0], [0, -2, -2, 0], [0, -2, -2, 0], [-2, 0, 0, -2], [-2, 0, 0, -2], [-2, 0, 0, -2], [-2, 0, 0, -2], [0, -2, 0, -2], [0, -2, 0, -2], [0, -2, 0, -2], [0, -2, 0, -2], [0, 0, -2, -2], [0, 0, -2, -2], [0, 0, -2, -2], [0, 0, -2, -2]] -- messages: 100141 nosy: ughacks severity: normal status: open title: bug in s.append(x) type: compile error versions: Python 2.6 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue8023 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue8023] bug in s.append(x)
Mark Dickinson dicki...@gmail.com added the comment: This is a bug in your code, rather than in Python. A simpler example, for the purposes of explanation: root, total = [0], [] total.append(root) total # good so far [[0]] root[0] = 1 # modify root total # note that total changes here! [[1]] total.append(root) total [[1], [1]] In effect, total contains two references to the same list. Modify that list, and you'll see those changes reflected in `total` as well. -- nosy: +mark.dickinson resolution: - invalid status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue8023 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue8023] bug in s.append(x)
ughacks ugha...@yahoo.com added the comment: Thank you for kind explanation. So s.append(x) just copies the pointer, not the actual value. It is a little tricky to know that. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue8023 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue8023] bug in s.append(x)
Mark Dickinson dicki...@gmail.com added the comment: So s.append(x) just copies the pointer, not the actual value. Yes, that's a reasonable way to think about it (though the term 'reference' seems to more popular than 'pointer' in this context). It matches the implementation, too: internally, a list is represented as a resizable array of pointers to objects. (For CPython, anyway; alternative implementations of Python might differ.) Implicit object copies are rare in Python. comp.lang.python is a good place to ask questions about Python's object model. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue8023 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
