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See partial work (DDD, TDD, brief stab at part of impl) here: https://github.com/pyinvoke/invoke/tree/assert_all_ran
See docs in that tree for the explanation. I didn't see an obvious and non ugly/fragile way of implementing it (the multiple possible values for mocking contexts make it kind of a nightmare) and I'm not sure it's SO useful that it's worth holding up other stuff that's already merged for the current release.