def _start_tracer(self):
        tracer = self._trace_class()
        tracer.data = self.data
        tracer.arcs = self.branch
        tracer.should_trace = self.should_trace
        tracer.should_trace_cache = self.should_trace_cache
        tracer.warn = self.warn
        fn = tracer.start()
        self.tracers.append(tracer)
        return fn

    # The trace function has to be set individually on each thread before
    # execution begins.  Ironically, the only support the threading module has
    # for running code before the thread main is the tracing function.  So we
    # install this as a trace function, and the first time it's called, it does
    # the real trace installation.
Start a new Tracer object, and store it in self.tracers.
dataset/ETHPy150Open RoseOu/flasky/venv/lib/python2.7/site-packages/coverage/collector.py/Collector._start_tracer