I agree that supporting non-stdlib types is in some ways a separate issue from
how to manage coercion with mixed stdlib types. Can you provide a complete
patch (e.g. hg diff > coerce_types.patch).
http://docs.python.org/devguide/

There should probably also be tests added for situations where the current
implementation behaves undesirably. Something like these ones:
http://hg.python.org/cpython/file/a97ce3ecc96a/Lib/test/test_statistics.py#l1445

Note that when I said non-stdlib types can be handled by coercing to float I
didn't mean that the output should be coerced to float but rather the input
should be coerced to float because __float__ is the most consistent interface
available on third party numeric types.

Once the input numbers are converted to float statistics._sum can handle them
perfectly well. In this case I think the output should also be a float so that
it's clear that precision may have been lost. If the precision of float is not
what the user wants then the documentation can point them toward
Fraction/Decimal.