def start(self) -> TTraceFn:
        """Start this Tracer.

        Return a Python function suitable for use with sys.settrace().

        """
        self.stopped = False
        if self.threading:
            if self.thread is None:
                self.thread = self.threading.current_thread()

        sys.settrace(self._cached_bound_method_trace)
        return self._cached_bound_method_trace
Start this Tracer.

        Return a Python function suitable for use with sys.settrace().

        
coverage/pytracer.py