Dear Python-Dev, I am the author of bidict, a bidirectional map implementation for Python. A user recently filed a bug that bidict should be a subclass of dict, so that isinstance(mybidict, dict) would return True. I replied that the user should instead use isinstance(mybidict, collections.abc.Mapping), which does already return True, and is more polymorphic to boot. But before I put the issue to bed, I want to make sure I'm correctly understanding the intended usage of collections.abc, as well as any relevant interfaces I'm not currently using (collections.UserDict? __subclasshook__?), since the documentation leaves me with some doubt. Could any collections experts on this list please confirm whether bidict is implemented as the language intends it should be? Some quick references: https://bidict.readthedocs.org/en/latest/other-bidict-types.html#bidict-type... https://github.com/jab/bidict/blob/master/bidict/_bidict.py I would be happy to try to capture what I learn from this thread and write up a guide for collections library authors in the future, or otherwise pay your help forward however I can. Thanks and best wishes. -jab