Again…, what “slow down” mean? Since it is just an additonal implementation and doesn’t share the implementation with dict, we can not use the fast path in the dict_merge. There are no slowdown here. How the basedict help it?
Naoki, please, or I code or I respond to you… anyway:
what “slow down” mean?
Try for example to re-implement dict using abc.MutableMapping and bench it 
How the basedict help it?
First of all, you have not to do all the things I already mentioned. Furthermore, IMHO the dict iter implementation (and also the dict views) should be more generic (this is also said by BDFL in a comment…) and have a dedicated .h and .c files… I mean, dictobject.c have ~4’700 rows o__0
This way, another dict-like object can use a more generic dict_iter implementation, and you can check against it.
Oh, didn’t you mean implementing dict-like in C and register it to ABC?
I never thought you are talking about implementing dict using abc.MutableMapping.
I am always confused about what you are saying…
Since you never show elaborate example, I gave up on understanding your idea. I will wait for your implementation. C is much easier than English.
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