New submission from David Farrar <[EMAIL PROTECTED]>: Using a python module that expected me to pass a dictionary as a parameter to a function, I found that I was getting unexpected results when using a class which inherits from types.DictType.
The module was passing the instance I had supplied as a parameter to update() on a newly created dictionary. I wanted my class to present its methods as items of the dictionary it was pretending to be. Even though I could see that __getattribute__ had been called to get 'keys', keys() was not called and I ended up with an empty dictionary. Trying to find the cause of the problem, I downloaded the python source package and found that in PyDict_Merge (in Objects/dictobject.c, line 1359), there is an optimisation that is run if PyDict_Check determines that the parameter we passed is itself a dictionary. The optimisation takes advantage of the fact that we know how data is stored in the dictionary and accesses whatever it needs directly. I had overridden keys() to provide the result I wanted but this optimisation prevented the code from ever being run, leading to code that seems like it should work failing with no apparent cause. The only way I could find around this was to raise AttributeError in __getattribute__ when getting 'keys', causing dict_update_common to call PyDict_MergeFromSeq2 in place of PyDict_Merge. Should the call to PyDict_Check in PyDict_Merge not be replaced with PyDict_CheckExact ? This would keep the optimisation in place in most cases, where we know that it is safe, still allowing keys() to be overridden as you expect. ---------- components: Interpreter Core files: dictionary.patch keywords: patch messages: 70353 nosy: david severity: normal status: open title: dict.update() optimisation gives unexpected/invalid results when passed a subclass of DictType type: behavior versions: Python 2.5, Python 2.6 Added file: http://bugs.python.org/file10997/dictionary.patch _______________________________________ Python tracker <[EMAIL PROTECTED]> <http://bugs.python.org/issue3458> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
