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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def rotate(self, source: str, dest: str):
 """ When rotating, rotate the current log. The default implementation calls the 'rotator' attribute of the handler, if it's callable, passing the source and dest arguments to it. If the attribute isn't callable (the default is None), the source is simply renamed to the destination. :param source: The source filename. This is normally the base filename, e.g. 'test.log' :param dest: The destination filename. This is normally what the source is rotated to, e.g. 'test.log.1'. """

        if self.rotator is None:
            # logging issue 18940: A file may not have been created if delay is True.
            if await self.loop.run_in_executor(
                None, lambda: os.path.exists(source)
            ):
                await self.loop.run_in_executor(  # type: ignore
                    None, lambda: os.rename(source, dest)
                )
        else:
            self.rotator(source, dest)