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I think we should certainly support asserts regardless of whether Py_DEBUG is in force (although Py_DEBUG should imply asserts run too).
And I wish you had stuck to just that much ;-) The argument against, e.g., 'assert(!PyErr_Occurred())', seems exceedingly weak. An `assert()` is to catch things that are never supposed to happen. It's an error in the implementation if such a thing ever does happen. But whether that error is in the Pytnon core or an external C extension is a distinction that only matters to assigning blame - it's "an error" all the same. It's nothing but good to catch errors ASAP.
Where I draw a hard distinction between assertions and Py_DEBUG is along the "expensive?" axis. The more assertions the merrier, but they better be cheap (and `PyErr_Occurred()` is pretty cheap). 
Py_DEBUG does all sorts of stuff that's expensive and intrusive - that's for heavy duty verification.
So, to me, 'assert(!PyErr_Occurred())' is fine - it's cheap and catches an error at a point where catching it is possible. Finding the true cause for why the error is set may be arbitrarily more expensive, so _that_ code belongs under Py_DEBUG. Except there is no general way to do that, so no such code exists ;-)