commit a walkers data after it is collected

def _commit(self, ref):
        'commit a walkers data after it is collected'
        path_times = self._weakref_path_map[ref]
        path_times.append(nanotime())
        del self._weakref_path_map[ref]
        path = tuple(path_times[1::2])
        times = path_times[::2]
        if path not in self.path_stats:
            # tuple to save a tiny bit of memory
            self.path_stats[path] = tuple([
                Duration(interval=False) for i in range(len(path))])
        path_stats = self.path_stats[path]
        for i in range(1, len(times)):
            path_stats[i - 1]._stats.add(times[i] - times[i - 1])