As a reminder, in case you didn't consider it, some code in your exec string might be run after the exec has finished, due to garbage collection.

    d = {"__builtins__": {"print": print}}

    exec("""

    def delay_until_gc():
      try:
        try:
          yield 1
        finally:
          print((1).__class__.__bases__[0].__subclasses__()[:3])
      except:
        raise

    it = delay_until_gc()
    it.__next__()

    """, d, d)
    del d
    print("Finished exec.")

  Finished exec.
  [<class 'type'>, <class 'async_generator'>, <class 'bytearray_iterator'>]

  import gc
  toybox(1)
  exec(..., d, d)
  del d
  gc.collect()
  toybox(0)

If you allow any mutable object into the globals or locals dictionary, such that the exec'ed code can attach something to it, then you can't even use gc.collect() to ensure the exec'ed code can no longer be executed.