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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.