[Python-Dev] Intended Usage of collections.abc for Custom Collections

[jab at math.brown.edu]

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-hierarchy
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
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