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Sorry, that's not accurate - you have enough code in b.py to make the relative import work by convincing the interpreter it's actually being done in a package.
So what you're seeing in that regard is the fact that runpy is not any kind of sandbox - it shares process global state, including the import system, with all other modules. While the temporary module will be reverted automatically by runpy, any child imports will always remain visible in sys.modules, and any other side effects will remain in place (e.g. codec registrations).