How do you ensure new insafe opcodes in Python do not cause problems? Or, how do you make sure new opcodes in Python which are safe do not cause drivers to fail?

Since the whole reason we have this is to keep people safe while allowing the majority of drivers to function, I think it's better to err on the side of caution. Any new opcodes that didn't exist before are likely to be for new features in Python. So, old drivers aren't likely to break, while new ones could be fixed on a case by case basis - assuming that the need actually arose.

(I'm guessing here though that these opcodes don't change much between Python releases, especially between things like 3.4 to 3.5. At least for the kinds of basic ops needed for the basic math equations used in driver expressions, it's unlikely that there would suddenly be a whole lot more to add or change.)

You can't predict how optimizer will place opcodes. It is possible that old expression will use new opcodes due to optimizer. Perhaps.

@Sergey Sharybin (sergey), while its possible new python uses opcodes differently.
the current method whitelists opcodes, so wost case new expressions fail in the future.
Since we control the Python version - we can manage this - and CPython are not changing this area very often.

One thing that *could* go wrong is a bytecode that currently safe, becomes unsafe.
(while this seems unlikely, its possible).