Behavior for mixed operations varies greatly between Python versions.

The first table over here lists the differences and should be valid
for decimal.py:

http://www.bytereef.org/mpdecimal/doc/cdecimal/index.html#floatoperation-signal


As an extension, cdecimal has the FloatOperation signal, which
enforces stricter semantics. The second table on that page lists
the behavior if the signal is enabled.