Fwiw, I'm probably -0 on the feature itself. Someone suggested it could be useful for xarray, but I'm not sure now what that would look like. If someone had an example, I could easily be moved.
agreed -- I can imagine the use case, but am not enough of an xarray user to com eup with a compelling example.
But if it IS useful for xarray (and maybe Pandas, and who knows what other nifty packages -- maybe a database wrapper? -- that doesn't mean it has to be used for Mappings.
Jonathan -- I'm still quite confused about what your proposed syntax with dicts means, could you maybe both:
describe it in words
Give complete examples -- that is, what the dict looks like before an after the operation, and what is returned.