New submission from Aaron Brady: Hi, I asked about the inconsistency of the "RuntimeError" being raised when mutating a container while iterating over it here [1], "set and dict iteration" on Aug 16, 2012.
[1] http://www.gossamer-threads.com/lists/python/python/1004659 I posted a patch on the ML but never submitted it. People's reaction seemed ambivalent. Now I have an idea for a different implementation. I'd like to take another shot at it. It's one of the worst silent errors, since there's an error in the *iterator* when we call a *set* method. We're going to add something to make it safer, at least in the sense of getting a clear failure, if the programmer does something that's always been ill-advised. We have a number of options for the implementation. We still have the option to introduce "IterationError", possibly a subclass of "RuntimeError". These options are still applicable: 1) Collection of iterators . Invalidate all "open" iterators on mutating . a) Linked list .. i) Uncounted references .. ii) Counted references .. iii) Weak references . b) Weak set 2) Version index / timestamp / "memo" . Iterators check whether the container has been mutated since they were created . a) No overflow - Python longs .. i) Reset index if no iterators left . b) Overflow - C ints / shorts (silent error) 3) Iterator count . Raise exception on mutation, not iteration The new option is: 2d) Use a dedicated empty *object* for a timestamp or "memo". A new memo is created on every mutation. Before advancing, the iterator checks whether the current memo is a different object than it was when it was created. Costs: The existing silent error is fairly rare. The container gains a pointer to its current memo. The iterator loses the cached length but gains a pointer to a memo. The memos are blank objects: a "Py ssize t" and a pointer with certain flags at time of writing. Speed is the same: comparing the lengths is replaced with comparing the memos. Some caveats: The memory manager is used to obtain perpetually unique IDs. A unique algorithm could be used instead of the memory manager, though the memo needs to contain a reference count more or less regardless. There can at most be one memo per iterator. The approach is outlined in pseudocode here [2]. Implementation could be optimized slightly by only creating new memos if iterators have been opened, shown here [3]. [2] http://home.comcast.net/~castironpi-misc/irc-0168%20mutating%20while%20iterating%20markup.html [3] http://home.comcast.net/~castironpi-misc/irc-0168%20mutating%20while%20iterating%202%20markup.html ---------- components: Library (Lib) messages: 224071 nosy: castironpi priority: normal severity: normal status: open title: Mutating while iterating type: behavior versions: Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4, Python 3.5 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue22084> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
