def _installation_trace(self, frame: FrameType, event: str, arg: Any) -> TTraceFn | None:
        """Called on new threads, installs the real tracer."""
        # Remove ourselves as the trace function.
        sys.settrace(None)
        # Install the real tracer.
        fn: TTraceFn | None = self._start_tracer()
        # Invoke the real trace function with the current event, to be sure
        # not to lose an event.
        if fn:
            fn = fn(frame, event, arg)
        # Return the new trace function to continue tracing in this scope.
        return fn
Called on new threads, installs the real tracer.
coverage/collector.py