> So sys.dont_write_bytecode is there to prevent bytecode writing but not loading.
Ah right.  Providing a way to avoid reading them sounds good.

> If you set sys.implementation.cache_tag to None you stop all bytecode usage (reading and writing)
This I feel uncomfortable with.  First, I would separate writing and reading; second, I think of sys.implementation as a read-only information holder, and would think it immutable (note that I haven’t read the latest version of the PEP); just like sys.flags.dont_write_bytecode does not change but sys.dont_write_bytecode can change.  So even though I think sys has a lot of attributes, the cleanest thing here would be a new sys.dont_read_bytecode attribute (as was proposed in the thread), maybe also with a new command-line option (or using -BB) and environment variable.