[issue6326] Add a swap method to list
Antoine Pitrou pit...@free.fr added the comment: Even if C++ decides it's an useful feature, it's not a sufficient reason to add it to Python! Based on previous discussion and the converging advice of others, this bug can IMO be closed. -- resolution: - rejected status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
Adam Olsen rha...@gmail.com added the comment: Fix it at its source: patch your database engine to use the type you want. Or wrap the list without subclassing (__iter__ may be the only method you need to wrap). Obscure performance hacks don't warrant language extensions. -- nosy: +Rhamphoryncus ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
Kristján Valur Jónsson krist...@ccpgames.com added the comment: fyi, here is the thread from python-ideas: http://mail.python.org/pipermail/python-ideas/2009-June/005042.html The parallel to C++´s future rvalue reference is interesting (http://www.artima.com/cppsource/rvalue.html, see the move application) Especially, perhaps, in light of this quote from http://evanjones.ca/rvalue-references.html, criticising the concept: ...Or add a swap member function. This is not elegant nor is it as general. However, this is what we've been using for decades. It also requires zero changes to C++ compilers, and requires users to learn nothing. It has been immortalized in books like Effective C++ (Item 25: Consider support for a non-throwing swap). This, incidentally, is where the name 'swap' comes from, me being an old C++ programmer. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
New submission from Kristján Valur Jónsson krist...@ccpgames.com: It is sometimes useful to be able to swap the contents of two lists and this patch provides a way to do so using the fastest way possible. a = [1, 2] b = [3] id(a), id(b) (100, 101) a.swap(b) a [3] b [1, 2] id(a), id(b) (100, 101) One application of this is to make help a performance problem when one wants to upgrade a list instance into a subclass instance. orglist = rawlist_from_server() mylist = ListSubclass(orglist) This involves copying duplicating the list, and then discarding, which can take a while for long lists. Much faster is using mylist = ListSubclass() mulist.swap(orglist) We are using this extension within CCP to decoratate database queries from our database engine, that are returned as python lists. The performance gained by shaving off this extra list duplication can be significant for large data sets. to change a list, into a -- components: Interpreter Core files: listswap.patch keywords: patch, patch messages: 89626 nosy: krisvale severity: normal status: open title: Add a swap method to list type: feature request versions: Python 2.7 Added file: http://bugs.python.org/file14341/listswap.patch ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
Raymond Hettinger rhettin...@users.sourceforge.net added the comment: One application of this is to make help a performance problem when one wants to upgrade a list instance into a subclass instance. Since this bypasses the subclass's __init__ and other methods, doesn't it risk violating subclass invariants? class CapList(list): def __init__(self, iterable=()): for elem in iterable: self.append(elem.upper()) class NoneCountingList(list): def __init__(self, iterable=()): list.__init__(self, iterable) self.nones = self.count(None) def append(self, value): list.append(self, value) self.nones += 1 if value is None else 0 def extend(self, iterable): for elem in iterable: self.append(elem) . . . IOW, a swap() method is problematic for some subclasses because it bypasses all of the subclass insertion/removal logic. The problem is compounded for subclasses written as C extensions because violating the internal invariants may lead to a crash. -- nosy: +rhettinger ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
R. David Murray rdmur...@bitdance.com added the comment: There are clearly enough subtleties to this proposal that it should be discussed on python-ideas first. If a consensus is reached you can reopen this ticket, referencing the discussion thread. -- nosy: +r.david.murray priority: - low status: open - pending ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
Raymond Hettinger rhettin...@users.sourceforge.net added the comment: FWIW, the technique is useful and fast, but it is complicated in its effects. I used something similar in the set_swap_bodies() internal code for set and frozenset objects but avoided exposing the behavior externally. -- status: pending - open ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
Kristján Valur Jónsson krist...@ccpgames.com added the comment: Indeed, I realized that it does allow overriding all kinds of behaviour and as such may be dangerous for the unwary. But isn't it possible to do so anyway? One way to increase safety would be to require that the other list is not a subclass (PyList_CheckExact(v)) so that no unexpected behaviour occurs for it, and so allow the 'target' list to simply override swap if it deems it unacceptable. At any rate, I thought I'd share this idea and accept that it needs discussion. I'll start a thread on python-ideas. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
Changes by Raymond Hettinger rhettin...@users.sourceforge.net: Added file: http://bugs.python.org/file14345/mview2.diff ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue6326] Add a swap method to list
Changes by Raymond Hettinger rhettin...@users.sourceforge.net: Removed file: http://bugs.python.org/file14345/mview2.diff ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue6326 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
