I think this kills the optimization effect for non-dicts. See on PyDict_Merge(). It takes the boolean parameter that controls the behavior in case of matching keys. I think the best would be to rename it to say _PyDict_MergeEx(), extend the boolean parameter to ternary parameter, and raise an exception if it is in the third state and matching keys are found. PyDict_Merge() would be implemented as a simple wrapper around _PyDict_MergeEx(). We should check wherever this affects the performance of dict.update().