[Python-Dev] Issue 19332: Guard against changing dict during iteration (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Thu Nov 7 13:29:36 CET 2013

On 7 Nov 2013 03:18, "Antoine Pitrou" <solipsis at pitrou.net> wrote:

Le 06/11/2013 06:41, Nick Coghlan a écrit :

The behaviour of mutating builtin containers while iterating over them is formally undefined beyond "it won't segfault" (one of the few such undefined behaviours in Python). The associated exceptions are thus strictly "best effort given other constraints". Not sure what you mean with "formally undefined". For example, you can perfectly well change a list's contents while iterating over it, and I bet there's a lot of code that relies on that, as in:: for i, value in enumerate(mylist): if somecondition(value): mylist[i] = somefunction(value) If you change "builtin containers" to "builtin unordered containers", then you probably are closer to the truth :)

I meant changing the length rather than just the contents of an existing entry. Even lists get a little odd if you add and remove entries while iterating (although I agree the sequence case is much better defined than the unordered container case).

Cheers, Nick. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20131107/85ef2016/attachment.html>

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