I would prefer one type too, if it can be done gracefully enough. I had thought of a set-mode function (method), but anticipate objection to such modal action-at-distance behavior. A context manager, which I had not thought of, somewhat alleviates that objection, though not entirely. If code written in try-except manner is imported and run under the context manager, it might or might not work. My toy example would, in that the final array might contain Nans rather than Nones, which would be all right since the user would anticipate that, having asked for that. But if the imported code used exceptions to switch algorithms, switching from exceptions to inf/nan might not work so well.

Consistently replacing 'return inf/nan' and 'raise FPException' (where appropriate) with "if flag: return inf/nam; else: raise FPException" seems not too difficult ;-). But yes, a PEP rather than more piecemeal changes.