I haven't completely following the type coercion discussion on python-ideas. but the statistics module at least needs a docs clarification (to explain that the current behaviour when mixing input types is not fully defined, especially when Decimal is involved), and potentially a behavioural change to disallow certain type combinations where the behaviour may change in the future (see
https://mail.python.org/pipermail/python-ideas/2014-February/025214.html for example)

Either option seems reasonable to me (with a slight preference for the latter), but it's at least clear that we need to avoid locking ourselves into the exact coercion behaviour of the current implementation indefinitely.