> I've tried think of other solutions, such as a generic wrapper for such functions or a helper to check whether an iterable is empty, and they all turn out to be very clunky to use and un-Pythonic.

So the main use case would be to detect an empty iterable in an efficient fashion? Something like the following code?

sentinel = objet()
avg = mean(data, default=sentinel)
if avg is sentinel:
   ... # special code path

Why not adding a statistics.StatisticsError subclass for empty set (ex: StatisticsEmptyError)? Something like:

try:
   avg = mean(data)
except statistics.StatisticsEmptyError:
   ... # special code path, ex: avg = default

Or is there another use case for the proposed default parameter?