Our unittest policy does not describe how to balance tests between python and C+, for code that is mostly or entirely C+ (with pybind11 wrappers). After this conversation on Slack, it was clear that the consensus is that tests of our C++ layer should be written almost entirely in python, with C++ tests only for purely internal C++ APIs or tricky things in the pybind11 layer (both of which should be rare).

Filing as a ticket instead of an RFC, given the clear consensus in the Slack conversation. I can make an RFC if someone does object.