It seems to me there is overlap between sys.implementation.cache_tag and sys.dont_write_bytecode.  I was expecting sys.impl.cache_tag to be purely informational, and not actually controlling some behavior. “If cache_tag is set to None, it indicates that module caching should be disabled”: isn’t that the job of sys.dont_write_bytecode?

(The difference that I see is that sys.impl.cache_tag is defined by the VM and dont_write_bytecode by each interpreter invocation; I was expecting VMs that never want to write bytecode to just always set dont_write_bytecode to False, i.e. make -B redundant.)