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The futures case is one example of a broader API design issue.
Note also that 'self' is similarly restricted. You might think these are 'corner cases', but to me it is poor API design. There may well be a reasonable case for passing 'self' as a keyword arg; what right has the API to dictate otherwise?
I think Python's private name mangling provides a reasonably clean, self-documenting solution, pushing the 'corner' case further to the corner (the restriction would then be on the much-less-likely mangled names).