[Python-Dev] Issue 19332: Guard against changing dict during iteration (original) (raw)
Victor Stinner victor.stinner at gmail.com
Wed Nov 6 16:10:16 CET 2013
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2013/11/6 R. David Murray <rdmurray at bitdance.com>:
On Wed, 06 Nov 2013 23:34:22 +1100, Steven D'Aprano <steve at pearwood.info> wrote:
On Tue, Nov 05, 2013 at 08:38:09PM -0800, Ethan Furman wrote:
> http://bugs.python.org/issue19332 Duplicate of this: http://bugs.python.org/issue6017 The conclusion on that also was that it is not worth guarding against such an unusual circumstance. If I remember correctly (and I may not) the size-change guards were added because without them there were certain cases that could either segfault or result in an infinite loop. --David
This exception is quite old:
changeset: 17597:32e7d0898eab branch: legacy-trunk user: Guido van Rossum <guido at python.org> date: Fri Apr 20 19:13:02 2001 +0000 files: Include/Python.h Include/abstract.h Include/object.h Include/opcode.h Include/pyerrors.h Lib/dis.py Makefile.pre.in Objects/abstra description: Iterators phase 1. This comprises:
new slot tp_iter in type object, plus new flag Py_TPFLAGS_HAVE_ITER new C API PyObject_GetIter(), calls tp_iter new builtin iter(), with two forms: iter(obj), and iter(function, sentinel) new internal object types iterobject and calliterobject new exception StopIteration new opcodes for "for" loops, GET_ITER and FOR_ITER (also supported by dis.py) new magic number for .pyc files new special method for instances: iter() returns an iterator iteration over dictionaries: "for x in dict" iterates over the keys iteration over files: "for x in file" iterates over lines
TODO:
documentation test suite decide whether to use a different way to spell iter(function, sentinal) decide whether "for key in dict" is a good idea use iterators in map/filter/reduce, min/max, and elsewhere (in/not in?) speed tuning (make next() a slot tp_next???)
More recently, I added another exception if a dictionary is modified during a lookup.
When I proposed a new frozendict type to secure my pysandbox project, Armin Rigo wrote that CPython segfaults must be fixed. So I fixed a corner case on dictionaries. http://bugs.python.org/issue14205
Thread in python-dev: https://mail.python.org/pipermail/python-dev/2012-March/117290.html
The discussion in http://bugs.python.org/issue14205 is interesting.
Victor
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