There is a difference.  An __json__ attribute would have to convert to a list first.  Adding support directly to the json module would allow the deque to be read directly.

I think you all are leaning towards premature generalization and making this harder than it needs to be.  Chris and Tarek's proposal is a reasonable and straight-forward, but it is not being pushed towards PEP territory and I think Guido would need to opine on whether to enshrine yet another dunder method that would infest the library and privilege the json serialization format over all formats.